max over at in hoc signo vinces has a post on constantine's vision and the sign which constantine was instructed to use as protection in battle, so i thought i might say a little something about it here.
the first thing that should be said is that the separate accounts of constantine's vision/dream must be kept strictly separate, for they do not recount the same event. lactantius (who has a dream, but no vision) includes instructions for the chi-rho at De Mortibus Persecutorum 44 (text; translation):
Commonitus est in quiete Constantinus, ut caeleste signum dei notaret in scutis atque ita proelium committeret. Facit ut iussus est et transversa X littera, summo capite circumflexo, Christum in scutis notat. Quo signo armatus exercitus capit ferrum.
Constantine was directed in a dream to cause the heavenly sign to be delineated on the shields of his soldiers, and so to proceed to battle. He did as he had been commanded, and he marked on their shields the letter X, with a perpendicular line drawn through it and turned round thus at the top, being the cipher of CHRIST. Having this sign, his troops stood to arms.
lactantius' dream seems to occur in the vicinity of rome, shortly before the battle with maxentius. eusebius' version in the Vita Constantini, in contrast, occurs before constantine marches his troops to rome, and includes both a vision and a dream. eusebius claims that constantine himself told him the story 'a long while after'; no vision is included in eusebius' account of the battle at HE 9.9.2-8. here is cameron's translation from VC 1.28-31:
About the time of the midday sun, when day was just turning, he said he saw with his own eyes, up in the sky and resting over the sun, a cross-shaped trophy formed from light, and a text attached to it whichsaid, `By this conquer' (τούτῳ νίκα). Amazement at the spectacle seized both him and the whole company of soldiers which was then accompanying him on a campaign he was conducting somewhere, and witnessed the miracle. 29 He was, he said, wondering to himself what the manifestation might mean; then, while he meditated, and thought long and hard, night overtook him. Thereupon, as he slept, the Christ of God appeared to him with the sign which had appeared in the sky, and urged him to make himself a copy of the sign which had appeared in the sky, and to use this as protection against the attacks of the enemy. 30 When day came he arose and recounted the mysterious communication to his friends. Then he summoned goldsmiths and jewellers, sat down among them, and explained the shape of the sign, and gave them instructions about copying it in gold and precious stones. This was something which the Emperor himself once saw it to let me also set eyes on, God vouchsafing even this. 31 (1) It was constructed to the following design. A tall pole plated with gold had a transverse bar forming the shape of a cross. Up at  the extreme top a wreath woven of precious stones and gold had been fastened. On it two letters, intimating by its first characters the name `Christ', formed the monogram of the Saviour's title, rho being intersected in the middle by chi. These letters the Emperor also used to wear upon his helmet in later times. (2)From the transverse bar, which was bisected by the pole, hung suspended a cloth, an imperial tapestry covered with a pattern of precious stones fastened together, which glittered with shafts of light, and interwoven with much gold, producing an impression of indescribable beauty on those who saw it. This banner then, attached to the bar, was given equal dimensions of length and breadth. But the upright pole, which extended upwards a long way from its lower end, below the trophy of the cross and near the top of the tapestry delineated, carried the golden head-and-shoulders portrait of the Godbeloved Emperor, and likewise of his sons. (3) This saving sign was always used by the Emperor for protection against every opposing and hostile force, and he commanded replicas of it to lead all his armies.
32 (1) That was, however, somewhat later. At the time in question, stunned by the amazing vision, and determined to worship no other god than the one who had appeared, he summoned those expert in his words, and enquired who this
god was, and what was the explanation of the vision which had appeared of the sign. (2) They said that the god was the Onlybegotten Son of the one and only God, and that the sign which appeared was a token of immortality, and was an abiding
trophy of the victory over death, which he had once won when he was present on earth. They began to teach him the reasons for his coming, explaining to him in detail the story of his self-accommodation to human conditions.  (3) He listened
attentively to these accounts too, while he marvelled at the divine manifestation which had been granted to his eyes; comparing the heavenly vision with the meaning of what was being said, he made up his mind, convinced that it was as God's own teaching that the knowledge of these things had come to him. He now decided personally to apply himself to the divinely inspired writings. Taking the priests of God as his advisers, he also deemed it right to honour the God who had appeared to him with all due rites. Thereafter, fortifed by good hopes in him, he finally set about extinguishing the menacing flames of tyranny.
thus, what constantine saw in the initial vision, according to eusebius, was a cross-shaped trophy with a text attached to it. when instructed in the dream to make a copy of the sign he had seen, a chi-rho is included at the top of the cross (of course, this too could have been a part of the initial vision which is simply not included in 1.28), but is not the sign itself; it is part of the sign. this cross-cum-banner-cum-chi-rho is the sign commonly referred to as the labarum. as cameron comments (p.210),
Eusebius saw the labarum in its established form, as depicted on Constantine's late coins, and here describes what he had seen later... . Even in this form it could be described as cross-shaped, and resembled a military vexillum; Firm. Mat., Err. prof. rel. 20.7 refers to it as the vexillum fidei.
on 'the shape of a cross' in 31.1, she comments (p.210) that
The whole structure is cruciform. The fact that the military vexillum was cruciform had been noted by Methodius, Porph. 1, who claimed that earthly emperors thus used the cross 'for the destruction of wicked habits'. The description of the wreath and the first two letters of the name of Christ point clearly to the later labarum, as it was depicted on coins.
and, finally, an interesting bit on the chi-rho itself (p.210):
Like other Christian signs, the chi-rho emblem is in fact rare on Constantine's coins, and the early silver medallions of 315 from Ticinum (Pavia) showing the Emperor wearing a high-crested helmet with the Christogram are exceptional (Fig.3). See P. Bruun, 'The Christian Signs on the Coins of Constantine', Arctos, NS 3 (1962), 5-35, against A. Alfoeldi, 'The Helmet of Constantine with the Christian Monogram', JRS 22 (1932), 9-23; though the form of the chi-rho is attested before Constantine, there is no certain Christian use (E. Dinkler, Signum Crucis (Tuebingen, 1967), 134-5).
i've not read any of these articles on the frequency and use of the chi-rho and have no position on the matter at present.