Here is a good review of 256MB vs. 512MB. But note that the computer has 1GB of RAM
. In the normal resolutions that is used by most gamers (10 x 7 and 12 x 10) it showed little improvments. But in 20 x 15 resolution, the extra RAM shows a lot of improvement, but only a few people will play games at that resolution.
Now in ultra mode in Quake 4, there is an improvement in performance. This is because id loves to put very detailed and high res textures in ultra mode. In FEAR they perform the same in normal and max settings but only on max settings in 16x12 res that the 512MB shows it's strength.
They concluded that on a 1GB machine, anything over 100MB of system memory usage starts to slow down noticably. This usually happens in very high resolutions that most gamers don't play at. But if you have 2GB of RAM there would be very little circumstances where there is sytem slow down.
By the end of this year, IMO, more VRAM will start to be used; but 256MB will still play games fine in lower resolutions like 10x7. 512 RAM does help in some cases, but isn't worth an extra 100 dollars when having 2GB will play it fine.
512 does not make much of difference at the present time, but Oblivion should give us a better idea of RAM usage in the future.
The 7800GTX 512MB beats the 256MB version in games that have the same memory usage (which is most games in normal res) becaue it has faster clock speeds, end of story.