get the ATI 5670. it more power efficient then the 8800GT and offers better performance then the 9600GT.
to answer your question, the 9600GT would be fine with your current PSU but you would need to use a power adapter that comes with the card. You would need a PSU upgrade for the 8800GT. Your CPU will be fine for any of the cards.
Hey, If I got the ATI 5670, would I just plug it in and play? Like, does it need anymore power to it other than just plugging it in? Also, does the 5670 come in PCI Express 2.0? Since I have DDR2 Ram, would it affect anything if the video card has, say, DDR3 ram?