Video Card Conundrum?

Alright, shopping around for a new Video Card, I currently have a Radeon 1800XL (has been a great card for the past 2+ years) and am on a budget of $150 ~ $160. Seems to be the consensus around the net that I should be looking at these three cards; GeForce GTX 460 768MB, GeForce GTX 550 Ti, and Radeon HD5770. It seems that the 460GTX is more favorable among the three but my concern is that it's 768MB while the others mentioned are 1024MB. Could anyone give me some input and/or advice on which I should go with, or does anyone have a suggestion for a different card?