Upgrade Path

My current system
Athlon X2 4400
7900GT 256MB
2 G Geil @ 2-2-2-4
MSI K8N Diamond 16
X-Fi Extreme Music
LG 19" LCD
Antec TP II 550 PS
Dual WD 250's 16 Raid 0

I want to upgrade for additional gaming performance. Currently my monitor is native 1280x1024 so I'm not pushing huge resolutions, just turning on all the eye candy AF, AA, etc. in games like BF2142.

So for that amount would it be better to upgrade the graphics card to say a 8800 series, or one of the new ATI models, or on the other hand upgrade the MB/CPU combo to say a Core2.

I see advantages and disadvantages to both. If I upgrade the graphics card at only 1280x1024 I may be bottelnecking the 8800 series cards with only a 4400 and only benefit from the additional VRAM and could be pushing the limit of my PS

The other route I land a faster MB/CPU, with potentially a better upgrade path once additional DX10 cards come out and drive the current market prices down (again prob need to upgrade my PS, and my RAM is probably going to be outdated)

So what's the best bang for my buck performance increase for this system? Additional info: I'm in the minority but I see a difference between 50 fps and 70 fps, and in huge multiplayer FPS battles I want steady 60 no matter what and would sacrifice picture quality to get it (i.e. even with the system I have now I still don't run BF2142 @max for this very reason). Budget is around 500 bucks.

You advice is appreciated. Thanks

do you think I'll need to upgrade my power supply then?

Yes it would be a good idea.