If you have a T&L capable video card with at least 32 MB of video RAM (such as nvidia GeForce 2 or better or ATI Radeon 7000 or better) then you need at least:
600 MHz P3 processor 
256 MB RAM if Windows XP, Windows ME, Windows 98 or Windows 2000 
If you have a non-T&L capable video card (such as TNT2 or Intel Integrated for example) then you need at least:
1.5 GHz processor 
256 MB RAM if Windows XP, Windows ME, Windows 98, or Windows 2000 
Recommended video cards (and the more video memory the better):
ATI Radeon 9800 Pro 
Nvidia Geforce 4 or FX series cards