To be honest, I've never dealt with that issue so I don't have a solid answer; but I'm pretty sure that your video cards simply won't operate to their full potential if they're not getting the recommended amout of power. There have been people running 8800GT's on 350W power supplies. It really all depends on how much power the rest of your computer needs, etc. If you have 3+ hard drives or cd/dvd drives or even 4+ usb devices that are always active, then you'd probably want a bigger power supply. If you're running with one CD drive and one hard drive, with minimal usb devices and a low power consuming processor, you're probably fine as long as the power supply meets the recommended 12V ratings (you can open your case and there should be a sticker on your power supply telling you what the ratings are). I would suggest not even buying one until you know if your power supply has the recommended 12V rail rating (just open up your case and check).
Oh, and another thing about the 9600's: they're a lot cooler than the 8800GT's. A lot of people reported 8800GT's running at 90C during a full load, the 9600GT is much cooler due to the card's core design.
