In every game under different testing methods we saw that 512MB does make a difference, and in some cases a large one. Quake 4's performance increases in Ultra mode were the most impressive here, but HL2: Lost Coast also showed some credible boosts. F.E.A.R was not so impressive but in real world testing 512Mb showed its worth, especially on a system with 1GB RAM.
We saw that most games in medium to high quality settings used far more than 256MB to hold all the texture data, and so the overspill is already happening you do need 512MB of VRAM to hold all the texture and geometry data in VRAM. The question is, does using system memory for some of this have an impact on performance, and the answer seems to be only in extreme cases where 100MB or larger spills into system memory.
It's so difficult to choose either way really. You could buy a 256MB card today and it would most definitely serve you well for a year or two, but on the other hand there is no reason not to get a 512Mb card other than price. The performance increase 512MB offers over 256MB is here today in game we all play and the gap will only increase with the next generation of titles, think UE3 and you'll get what I mean.
I should also point out that this article assumes we are talking about cards like the 7800 GT or higher, don't waste your time with cards from the mid to low end, the extra memory won't give you anything.
So, should you buy a 512MB card? Probably, but you'll have to look hard at the data in this article to decide that one for yourself. Personally if I were looking for a new video card to last me a few years I would not even consider a 256MB card. If you're like me and seem to have a new card at least once a year or less, don't worry too much, 256MB will do you fine
