I think it's a slight bottleneck, but nothing to worry too much about.
In my estimation, a 9600GT is pretty well matched with the 6000+. An 8800GT is a little stronger, and your CPU is a little worse than the 6000+, so there lies your slight bottlenecking.
But no, I don't think it's worth the money to buy a new AMD x2 just to slightly unbottleneck that GPU. Save up and go at least quad core for your next upgrade. If your current mobo doesn't support quad core and you're dying to upgrade, don't buy an AMD mobo and AMD quad...buy Intel. That pains me to say as an AMD man, but it's true...
Unless you're on a budget. AMD still can say that it's good bang for the buck.