Your CPU socket and AGP is a dead end, I don't think there's much sense in putting more money into a dead-end system.
What exact power supply is that? or if you can tell me if it has a 24 pin main connector? You might be looking at a new power supply too.
You'll need at least a 7200rpm drive...a 10000 rpm drive speeds things up some, but it won't help with gaming performance.
I would look at a Socket 939 system so you can re-use the RAM. First though, what is your budget?
If the budget is a little tight, you might look at this:
http://www.newegg.com/Product/Produc...82E16814130274
Then again, you could keep saving until you have enough for a new build (or mostly new).