If you can solve your ram issue, IMO it might be worth keeping your current mobo/cpu anyway.
DDR3 doesn't offer vast improvements over DDR2, and going from 1066mhz to 1600mhz won't be much of a performance jump either.
The biggest improvements would be the 720 to the 3220, or the 6870 to the 660. Honestly in a gaming machine, I'd be tempted to up the GPU over the CPU, especially when your CPU is already a quad @ 3.2Ghz per core