amd dual core @ 2.7Ghz
8800 GTS 512mb
and is getting 15-20fps regardless of what settings he uses.
2.1Ghz dual core
and is getting crap performance on even the lowest settings
Just to point out a couple.
The PC forum over at gta-forums is littered with people complaining of similar issues. Mainly revolving around GTX260 and HD4xxx cards I've noticed.