Need help and opinions on new hardware

[Simulated-AI]

Hi all.

This is my actual rig:
Vista 64
Striker II Formula
Q9550@3,2Ghz
GTX285
4GB corsair pc 6400
X-Fi Extreme Music

But its not new anymore, and i think of replacing some hardware.

First i was thinking getting a GTX580 over my GTX285, because it would noticely increase my graphic performance. (and its easy).
Then again, why not get a new motherboard and a i7? ofcourse with new DDR3 ram. Then the bottle neck for the graphic card should be eleminated, if there would be any?

I could also use my GTX285 as a dedicated physx, but that would probably require me to upgrade my 620W Seasonic to a 750-850 Watt.

I would also say that my 4x1 GB ram is not identical! that means i have two different pairs that slows my bandwith due to not using dual channel optimal.

Overall i would say my computer is usefull, but i would like some kind af upgrade in the nearest future.

I hope to get some usefull replies, and all help/opinions are welcome.

Thanks in advance.