Tim Sweeney from Epic Games announced that in Unreal Tournament 3 and Gears of War for the PC you'll get a significant performance gain if you have a graphics card with 512MB rather than only 256MB:
As you can read for yourself, if an entry-level 8800 is your thing, skimp the 199-something card and just shell out additional 50 somethings for a 512MB card. In Blighty, 512MB board should cost 149 quids (yes, getting ripped off all over again), so this is the card Tim wholeheartedly recommends as "at least". However, with several partners coming down the line with 1GB cards, know that UE3 engine will more than welcome more memory. We guess this puts the whole idea by some engineers about "what's the use of 1GB memory with 256-bit interface" to rest. Yes, it is worthwhile. Nuff' said.
"Are you broke yet? If not, then go for more memory, especially if you plan to build mods or levels (as the tools are more memory-intensive); or if you plan to move to 64-bit Windows Vista, which manages memory more efficiently than 32-bit Vista or XP."
Second in the line for an upgrade is system memory. Just like we expected, there is no need to go overboard with CPUs, dual-cores or multi-cores will do just fine - but memory, memory, memory. Try to kill the need for swap file. If that means going to 4GB of memory, go to 4GB of memory and look at advanced operating systems that expand over that 32-bit limit of 2.75 to 3.24 GB of system memory.
"And, if you still have funds left over after that, and you’re feeling brave, then look at SLI, crazy cooling options, etc."
After a strong GPU and a lot of system memory were taken care of, then you can take a look at multi-GPUs.