Thinking about upgrading - some advice (old build inside)

GPU : VGA EVGA 512-P3-N872-AR 9800GTX

Mobo : MB MSI P45 NEO3-FR 775 RT (nothing special just DDR2, quad core capable - NO SLI capability)

HD : SATA 500Gig drive

CPU : CPU INTEL|C2Q Q6600 2.4G (Standard Q6600)

PSU : CORSAIR|CMPSU-650TX 650W

Ram : 4gig DDR2

--what i'm tryin to figure out is how much/what can i upgrade on the PC to be cost efficient. Do i upgrade the video card, mobo, or cpu. Should i upgrade the Mobo for something that can support SLI but can still use my CPU and perhaps DDR3? Should i just replace the video card and forget about SLI?

All tips would be appreciated, i'm willing to spend probably around 300-500 range.

I built this comp myself originally two years ago and am trying to figure out if i can get away with my cpu for another year or two, which if i can, i can just simply upgrade the video card. But then again i think DDR3 ram would be nice.

Thanks!