For GTA up to now my kit is great. 
AMD 3000+ Chip w/ 1 gig ram 
40 gig boot drive (OS & Drivers, User files) 
200 gig data drive (Games, MP3s & backups here) 
---  If you can afford the second hard drive, do it.  you can a lot of headaches if you have to do a wipe & reload. 
DVD r/rw + backup cd r/rw 
Soundblaster live (older) + Altec Lansing 5.1 (not in use at the moment, half my speakers disappeared in the last move)  
N-vidia 5500 w/356 meg 
XP pro, service pack 2 (I think they finally fixed it, installed SP2 a month ago) 
AVG free.  Download form AVG site, supported w/ regular updates.  If you can afford the pay version, it has extras. ---  Between this & Windows Defender, I haven't had any thing get in. 
All my GTAs fly on this machine & most of the components can be had reasonable (possibly cheap if you have access to used parts). 
Godfather, Mafia, Unreal tournament, Lego Star wars &  also look & run great.  Scarface, not so much... 
If you're on a budget, this kit should play most of the current games. 
Good Luck