I bought the one with extra memory because I play mainly games like GTA IV and Far Cry 2. Those sorts of games I thought would use a whole bunch more memory than any other game, GTA IV proved it, but no other game is yet to use more than 900mb video memory.
GTA IV on maximum settings for me uses about 1592mb of that 1792, so it was sort of worthwhile, I just need 64 bit and then I can go mad with memory. Also GTA IV I think is fine, never goes over the limit of my 2GB, it's pretty close though, most of the time in game it gets to about 1930-1970MB memory usage, so I still have another 70 or so left.
Anyway I thought that's how the memory limitation worked, Someone told me it only counted system memory, but I told him that it counted all memory in the computer.