All you really need to do is buy a new processor/motherboard that will run with the DDR II ram you have, and sell that graphics card, and buy a new one.
You could re-use the RAM, and the case, etc.
So, buy a AM3 motherboard, and a Phenom II X4
The only issue then though is, if you want it future proof, because, it'd be best opting for a motherboard that supports DDR 3 ram in that case, but if it was a rig that was going to stay with you, and you'd only want to upgrade the graphics card over time, and not the processor; you could do it that way, and get a cheaper motherboard that would do everything you wanted, now.
It'd make it a much cheaper build, and you could sell the individual parts, like the processor, and motherboard, but keep the rest.
That is how I would plan it out.
AMD Phenom II X4 965 Black Edition
The motherboard, I'm not too sure about, because like I said, it would be best opting for DDR 3 ram, which may only be available to you, using a processor like the one above...
People will be here to help shortly I'm sure, but if you were to sell the RAM also, you could get a motherboard like this:
Fully upgradeable motherboard:
Or, go for a cheaper motherboard that may not be as future proof, but still support DDR 3 ram...
Total so far $320.98...
Its up to you, but I would probably opt for a cheaper motherboard that may not be as future proof, so you can spend more on the graphics card...
I'm hoping people will be here to help in a sec