yea 1440x900 is my res, and just about everything on high settings (except crysis with x8AA and gtaiv, but gta was ported horribly to the pc anyway)
To what I understand, the memory size is really only crucial for multi-displays and large displays. 512MB can handle 20in or less very nicely. I looked long and hard at the 4850 vs the 4770, and the conclusion I kept getting was that there won't be a noticeable difference between the two. Same with the GTS 250.
And, no, the bigger the better doesn't really describe video card performance very well. The 9600GT might have a higher number, but it's not as good as the 8800GT. The 4830 is not as goods as the 4770. And so forth. Think of all the series as teams that have bad players and good players, in which there are some good players on a lower team who are better than the bad players on a higher team. Though, it's still confusing and more complex than that.