First you tell what do you want the GPU for?
And I strongly recommend a 4670/9600GT over those two you mentioned.
4650/9500GT are crappy performers due to their slower DDR2 memory.
The ones I mentioned have DDR3 memory and deliver solid performance for price.
You will find them in 4000-5000 Range.
9600GT may be a bit more expensive but they are well worth the money.