I'm actually about getting "literally" same specs rig.... as far as the hd 5850 you can't go wrong with this one. In the future you can do CF and you will still rock and roll but did you thought about XFX company, cuz they have LIFETIME warranty for their GPU... so just in case this one will fry you have no problems...
Btw I love this spinQ its awesome
and don't worry about psu its perfectly fine... actually on this 600 according to ATi you could run CF just FYI.
600-650W should be sufficient. Make sure that the PSU is 80+ certified & has Active PFC. This Antec Earthwatts 650 is $80 with free shipping. Or the Corsair 650TX is $90, has a 10% off promo code ending today, free shipping and a $20 MIR. You could also step up to an 80+ Bronze/Silver/Gold if you want, though those are typically going to cost a little more than the two I just linked. For example, the Antec TP-650 is $100 with free shipping.
Good PSU brands tend to be Antec, Seasonic, Corsair, Silverstone. If you are interested in a brand that's not on that list, I'd recommend looking for a review of it on www.jonnyguru.com.
I've been looking at this board that would allow me to do Crossfire in the future. Problem is, I have no access at all to a X4 chip that I can drop in to do the necessary BIOS update so I can use the X6. What the heck do I do?