1. What model/make PSU u have?
2. Your mobo has a PCIex16 slot and gen 2.0/2.1 is backwards compatible with 1.0/1.1 lanes
3. I would say your best gains/$$ would be either hitting a sub $100 HD 4850 or hunt used GTS 250/HD 4850/HD 4770 in the 2nd hand hardware market on the cheap hehe
I would recommend to upgrade your RAM to 4 GB so that would help your cpu, then I would probably go for the 4890 or a cheap Geforce GTS series. but i don't think the 4670 series will be worth to spend on them. Keep on mind if you are planing to change your cpu then it depends on what you will get, you 'll have to change MB/Ram