I bought my 8800GT second hand last december for £80 from ebay as I couldn't find one in the shops.
My GTS250 is essentially a die shrunk, over-clocked, cooler, faster, possibly even lower power, 8800, doesn't require a disabler to fix smearing in QT either. It was £82 a few weeks ago.
Choosing an 8800 because it was in a mac doesn't really seem to make sense unless you flash it with a mac rom too, and then presumably you loose the ability to dual boot into win or linux?
It's very slightly more complicated to generate the efi string for the GTS250, but really no big deal. Basically I wouldn't limit yourself to such an old card, there is no great advantage. The newer ones are less power demanding, run cooler
and work just as well.
Budget is a different consideration of course
As for your power supply, I use a 400Watt PS so yours should be okay, but I don't feel qualified enough to give you an accurate assessment and therefore a 100% guarantee that it's going to work. So if anybody else here can help then please chip in.
Well, my impression is that when the system runs low on power it gets unstable. Things will crash, you'll get kernel panics etc. And because this is a hack you'll have no idea whether its software or hardware that's causing the problem. Giving yourself some head room with the power supply is wise. And there is a difference between 350 and 400 watts in case you hadn't noticed
It'll probably be ok, most of the time at least, particularly if you can get a less power hungry graphics card, only have one hard disk and don't use any other add in cards or a ton of bus powered usb peripherals.
P.S. My first reply was longer, but I got a KP after starting virtualbox, the first real KP I've had on this hack