Doom3 on Ultra settings (although yes it can be run I'm not saying it can't) says it's recommended for 512mb cards which have yet to be released
So I think the newer games like far cry, doom3, HL2, all the new really taxing games probably will use that 256mb of RAM.
I already told you what happens when the VPU runs out of memory. It simply supplements with the systems memory.
If you've ever looked in your BIOS and seen something that says 'AGP APERTURE SIZE' it's referring to the amount of system memory that will help out in the event that the graphics card runs out of it's memory....if you have 256mb of system RAM I suggest you put it to like 64mb aperture size, if you have 512mb and up then I'd say 128mb is a good aperture size.
I have a Sapphire 9600XT 256mb video card I'm trying to get rid of. I'm only asking $115 because most of the $15 in that will go for shipping, but the main reason it's so low is because FedEX REALLY screwed up the box...when it arrived the box was destroyed, the drivers cd was smashed and the metal on the card is bent a little...the overall card has a slight bend to it, but it still works
If you're interested let me know...I know it sounds like it's garbage from my description lol, but I plan on putting it in my secondary machine and fully ' test driving ' it to make 100% sure that it's a perfectly working card.
If I don't get it sold before christmas, then I have a friend who's willing to take it off my hands, he just won't have money for a while and I'm trying to get gifts for the family and such.