If your video card is 3.3v (very old), it won't fit new mobo which accepts 1.5v.
Forget Sempron. Go for Athlon XP 2500+. Mobo would be any nForce2 chipset: ASUS A7N8X-whatever, Abit NF7-S V2, DFI NF2 Infinity, etc. etc.
Forget Sempron. Go for Athlon XP 2500+. Mobo would be any nForce2 chipset: ASUS A7N8X-whatever, Abit NF7-S V2, DFI NF2 Infinity, etc. etc.