Ehm I think it's your graphic's card that is your weakness. Thought im not sure what equivalent to 2900 nvidia got, but that's the only reason i can think of.
Are you running vista 32 or 64 bits? I read in another thread about a guy with more ram then the computer could handle, making his graphics ram unused(slowing down his computer). For what i have understood 32 bits only supports a maximum of 4 gig ram+graphic ram.

Those are my thoughts and I hope they will be to any help for you.