IMO you should atleast throw a Quad in that board of yours you will be able to get another 2 years out of your rig with a decent core 2 quad and a nice GPU. But this all depends on how much you are willing to spend. For around $250 you can get another couple of years out of your rig or you can do a totaly new system and spend around $450 to $500 and still will probably have to upgrade in a couple of years.
^ I think for $350-400 OP can get a new AMD AM3 set up (just CPU + DDR3 RAM + Motherboard)
I would say he would see better results with that $300 to $400 by sticking with a core 2 quad and DDR2 800 and bulk up on his video card with that kind of money you could get a 5830 and a Q8xxx or even cheaper a Q6600 which would have no problems with games for the next year or two but it could go either way. Because In reality he would really be better off spending the money on an I5 750 or something which hopefully will drop in price with the release of the Thuban. Thats atleast what I am holding out for LOL.
^ Don't know if OP's a gamer or what his current GPU is. Right now, a new C2Q is $170 and 2x2GB DDR2 800 or 1066 is ~$80 or so. That's a total of $250 right there.
He specified in another post that he is trying to upgrade for gaming he currently has a P5KPL-VM motherboard and AND 4 GB of DDR2 ram thats why i suggest just upgradeing to a quad and get a nice GPU.
Since SC2 is not out yet, I'm going to wait till last minute to upgrade, maybe to LGA1156 or LGA1366, with i3(cheapest CPU) and hopefully they'll drop in price by then.