No SLI does not add video memory =( i wish i was that way, It just takes some of the Load off the GPU like one will take the Load for 3D and the Other for 2D, another reason i never went back to SLI it's cheaper to Buy 1 better 3D card then it is to Get SLI and the single card will run better.
But yea if you only have like 2gig of ram even if that is DDR3 ram you need some more my friend =P
with running win7 it's kind of like WinXp but it's still slower unless you have at best 4gigs of ram but for games i say Get 8gigs of DDR3 ram and have Win7 64bit installed or have that much ram an't doing you s**t

it's the same thing i told my cousin He has a New PC Core i7 920 with 8gigs of DDR3 ram and was going to run Win7 32bit and i was like NOooo you need 64bit man lol with that much ram you have to install 64bit or your just wasting your money.