1)Minimum requirements will never be DX10. Perhaps recommended, but not minimum.
2)
Xpress is just on-board, right? No. I'd say nVidia 7600 or ATI equivalent
Pentium D, Athlon 64? You nuts? Athlon doesn't even exist anymore and Pentium D is even worse then P4.
3)A game like GTA takes lots of RAM. And the PC version functions differently. If you have Vista you'll at least need 2 gig.
4)Do you know any computer with 600 MB RAM?
5)
You will not need 5.1 or a sound card at all.
Recommended:
Windows XP. Vista to benefit from DX 10
RAM: 1.5 on systems running XP. 2 gig for Vista
CPU: Core 2 Duo E6400 or AMD equivalent
Free Space: 9 gig free space on HDD
Graphic Card: nVidia 7600 (for DX 10 8600 GTS) or ATI equivalent.
VRAM: 256 MB, 320 recommended.