You won't find RDRAM anymore.
1)You need a complete system upgrade.
2)Look for second hand RAM if you still want it.
3)You may end up spending more than what it takes to upgrade the CPU+MB+RAM.
Sell your current stuff and you could get better stuff for as little as 3-4k investment, though 5-6k+ is recommended.
AMD 64 3000+ Venice
Foxconn nforce 4 with onboard video
2 X 256MB DDR 400
All this should cost around 9k and after selling your previous stuff you should be able to reduce maybe 1-2k.