Since you have the new 80gb HD, u have plenty of space for Dual boot. I would dual boot Win 2k with Win98. Use Win2k for everyday tasks, 98 for gaming. I had that setup for a while till I moved to XP. XP probably is not a good idea for you, as the processor might be a little lacking.

Of the new video cards, I like the ATI radeon 9000. Can be had for 70 USD if you know where to look. Its faster thena GF3, but slower then a GF 4 (TI).

Personally I run a GF2 with Athlon XP 1700 @ 1.75ghz.