Very, very few games could really benefit from using a 512MB graphics card; most, even with the highest settings and AF, (AF tends to have a bigger impact on VRAM usage than resolution/AA) you won't really pass 256MB of video RAM usage. In the few cases where it occasionally goes a bit over 256MB, the bit that's stored in the main system RAM, and shuffled back to the video card, is generally of little enough significance to bring any meaningful penalty to performance.
One game that DOES seem to have an effect, though, is Oblivion; at high textures and x8 AF, it will actually pass 256MB of video RAM used, but even then, won't really have much of a performance penalty. What REALLY makes a difference, though, is once you start piling on all those texture-replacement and other quality-enhancement mods; for those cards that support it, there are, for instance, LOD-texture replacements that are at 4096x4096 instead of the stock 1024x1024. Plus, the game, by default, uses low-resolution normal-maps on most surfaces, (resulting in things like "JPEG'd" faces) and there are mods that replace those. Once you pile them on, it's actually possible to even break the limits of a 512MB card, making a larger buffer such as found on the GeForce 8800 series kinda necessary. (and in turn, I think it's possible to exceed the 768MB VRAM of the 8800GTX...)
Sadly, there are no benchmarks availible for this, as all Oblivion benchmarks do NOT use mods or tweaks, and most of them don't even put the settings at 100%. (yes, that's right; most high-end cards won't be able to get 60fps at 100% settings at 1280x1024+ in that game)