HD 5850 if you can find one. I just ordered one, but it was $280 + $8.50 shipping.
Definitely the most $$$ I've spent on a video card. I expect it to last me a few years since I don't game as much as I used to.
I believe a 500w PSU is recommended, but it also depends on what your PC specs are and if you have a good quality PSU. Many times what ATI and nVidia recommends are more than what you really need because they want to make sure that your entire system has enough power whether you are have an E8400 or i7 920 CPU (and other components).
it depends on your PSU.... What spec your PSU? Go for HD 5850 only if your PSU can handle it... If it doesn't then maybe 4850 seems fair enough...