heres my suggestion for upgrades

1. get 2x ram sticks (2 512's or 2 1gb's) and set up dual channel
2. upgrade video card to a 6800gt or a 7800 or even the newest card next year & fans (u can then oc ur processor)
3. new processor (a x2 or FX depending on how gaming uses them in the future)
4. SLI ur card & a new PSU if needed
5. a new monitor unless u really need new HD's