That would depend on the video screen resolution you are running. If you are running multiple screens at high resolution, then it is widely known that more powerful GPU adapters scale up better to these larger resolutions.
1920x1200 (or 1920x1080) will run on 1gb Vram perfectly fine. Unless the card is old, the GPU and ram in the PC will cope with the very slight lack of Vram the graphic card has.
For instance, DOW2 on my GTX 260, which is old, has 720mb or so Vram, can run DOW2 on max settings with 4xaa and 8xaf, perfectly fine. In fact it runs smoother than my GTX 580 which has a bit of a stutter sometimes.
Is the FPS god like? No. But you don't need tons of Vram for single screen if the the rest of the PC can cope.
Now it is true if all the people with 1gb GC could add on another 200mb of Vram they would see an increase in FPS, mainly on their minimum FPS....But it will be a small change.
On triplescreen, it starts to heavily matter, since the entire PC, and the GC are going full wack struggling to display everything on the monitors, and lacking Vram will not be able to be counterbalanced by the rest of the PC.
So to summerise, Ratbuddy is correct.
For the perfect experiance, you want, a GTX 460 1gb or better. Or an ATI card as good or better. 4-6gb ram, windows 7 x64, an i7 first or 2nd gen clocked at 3.2ghz or faster (Or AMD with the same clock to clock speeds). Oh and to run the game in DX10, x64 with basic cfg settings to improve performance.
Thats not to say a system with worse specs wouldn't be able to run the mod, but don't expect to run the game on good settings.