Actually, you MIGHT be better off with the graphics card with more memory on it.
That memory is used to store and swap textures onto your display, thus the more texture memory you have, the less stutter you should have....
Played this demo on multiple PCs (2 of my own) and it ran great for me. Stutters here and there when entering new areas, but otherwise runs ok on a tweaked recommended settings.
BTW, the two systems i run it on:
Athlon64 3800+ (Venice core, read SSE3)
2gigs CorsairXMS
Radeon X800XT 256MB (AGP)
2x WesternDigital Raptor (74GB) RAID-0
2x WesternDigital 1200JB (120GB) RAID-0
Hyundai 19" LCD display (8ms response)
And:
Athlon64 3400+ (socket754)
1gig Kingston HyperX
GeForce6600GT 256MB
WesternDigital 80gigger
Viewsonic Optiquest G90F 19" CRT
Game looks about equal on each monitor, and that LCD i nabbed was a GREAT buy from Newegg by the way, LOW response for something that size, and the colour is nice and vibrant, and no issues with brightness...