I'd say your next upgrade should be a new mobo and CPU. An AM3/DDR3 mobo and a Phenom II x3 720 would be a great option, but you'd also need DDR3 RAM. You could upgrade to a DDR2 AM2+/AM3 motherboard, but then you'd have to upgrade that next time anyways, and by then, you might not be able to sell your DDR2 RAM for very much.
I'm not sure I would recommend the AM3 route for you. the change to ddr3 won't give you that much. Plus you're talking new mobo new ram, added cost...
How about a nice quad core? Pretty sure you could drop a PII X4 940 in that mobo and get 3.4-3.5 ghz with the stock cooler. I believe that mobo can handle 140W processors. It will definately outperform that X2. I thnk its about 190US.
4gigs of ram is plenty, although if you go with the Phenom 2 I would recommend 1066Mhz memory to get the most out of it. $50
I do recommend you look at a storagedrive
Those will give you the most boosts w/o a new mobo. in that order.
On the other hand, if you get a crossfire board, two hd4850's in crossfire would be nice, but i didnt see a monitor listed.