I have the base 27 inch- it was over £300 UK to upgrade to one with a 680 in it and given I do most of my gaming on Xbox and intend to buy the new Xbox this year, it simply was not worth it to upgrade. That money will pay for the new Xbox.
However, the 660 card is pretty good. Some games like Dirt 3 I can play all the top settings at high resolution.
Stuff like Witcher 2 and that new Assassin game on the App Store (1.99 - good game for that with good graphics) play good at high levels as long as the resolution is set to 720p. The screen on the 27 inch handles 720 p very nicely even though it is not native res. The quality of the graphics is substantially above what a current gen console can pump out.
So unless you want top resolution on the most demanding games, the 512 meg is fine- the lack of memory in it only makes a difference if you really want to up that res. if you are that bothered about gaming you'd be better buying a PC at half the price with more capability. The games you are talking about should be no bother at native res I imagine.
