What part should i upgrade first?

Ok, so i'm thinking on upgrading my rig but i don't know where should i start.

Here the specs of the PC:

MOBO - AsRock Conroe 1333-D667, 1 PCI-E 16x, doesn't support quad-cores, horrible for OCing...
CPU - Intel Pentium Dual Core E2180 @ 2.4GHZ, this isn't actually a problem because i'm only thinking on upgrade it when nehalem is out.
GPU - XFX GeForce 8600GT, is good for most of the games but not at my native resolution (1440x900)
PSU - A generic AOP (or something like that... :P) with 600W of power (i don't know if they are real or anything)
CASE - A horrible ventilated generic AOP case with only one fan at the rear.

So what do you guys think? my budget for this is only $200 and then i would have to wait at least three months to collect the money again for other upgrades.

I was thinking on a 8800GT but the PSU couldn't be enough and there's the problem with my case which could overheat the card. I honestly don't know where to start, your advices and help will be much appreciated.

2GB of DDR2 667MHZ, i think is pretty average...rigth???