It depends on what res you like to play at, and/or how big the game textures are. When I bought my first CF setup (X1950 pro's) they were 256MB frame buffer. Good enough for my 17" CRT. But when I upgraded to a 24" Syncmaster (1920x1200) things weren't so rosy anymore, heh.
The HUGE texture packs I like to run on HL2 (Fakefactory), Oblivion (Quarl's) And High Def mods etc brought my lil X1950's to thier knees. I see the game company's moving forward will adopt larger, better textures that will require larger frame buffers in the future.
You won't notice a differance in benchmarks that don't take advantage of the larger frame buffer, but the ones that do show a marked improvment in FPS.
Some games will respond differantly to more GPU frequency while others .Ram It all depends on how the game was written. Look at Carmacks Large textures in ETQW for example. It doesn't requre a lot of ram due to the way it 'streams'.
Games that are source based (HL2, L4D etc) however do benifit from more memory on the GPU. But in the future I think moores law applies to any scenario.
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