I'm guessing you have a silver HP Pavilion sitting on your desk infront of you right now. If not, then this conversation got really awkward. Anyway, on one of my computers I have a 4600+, which runs at 2.4GHz and is very much like yours. I have it running with a 8800GTX. I had a GTS250 on it for a long time, but the CPU would bottleneck the GPU, so I brought my 8800GTX from the dead (given I have a upgraded power suppy and case on that computer)
Anyway, with a budget like that, I would go with one of these:http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=602622&CatId=7005
It will run on your power supply and wont bottleneck it. My CPU in my other computer is actually not that slow, just don't put so many programs on it.
(That card also comes with a free game!)