What Component Should I Upgrade

Hello PC Enthusiasts! I'm considering upgrading my current system and using the leftover parts for a slower comptuer for my son. I mainly game (high res), use MS Office and email. I don't plan on going to Vista until next year or maybe this year if one of the newer DX10 games looks appealing. My system plays everything very smoothly at max settings, i.e. Oblivion, Bioshock, etc. so I'm not entirely sold on starting over just yet. I"m not sure if my board will support the Opterons as they are not listed as supported on the ECS website. Your recommendations are welcome.

My setup:
AMD 4000+ 939
1x BFG 8800 gtx oc
ECS KN1 Extreme SLI
2x 1gb Kingston Hyper X DDR-400
Antec HE500
Western Digital Serial 80gb 7200rpm
XP PRO SP2
Samsung 22" widescreen LCD