The motherboard and CPU are not compatible. If you want to stick with a sandy bridge processor then you will have to wait until mid-March at the earliest before 1155 (P67 and H67) motherboards become available again.
For gaming the i7-2600 has no real benefit over the i5-2500, so if I was you I'd 'downgrade' to the 2500 or 2500K if you want to overclock, or have that option.
Do you need a power supply? To run just one 5850 you would be fine with a good 500W. If you want the option to crossfire (which your current motherboard isn't really capable of - it certainly isn't worthwhile on it) or other upgrade options then a good 650W unit would be about right.
Oh and fill this out It'll help us forumlate a build. But assuming this is purely for gaming and your not overclocking.
i5 2500 (220$ish)
Asrock Extreme4 P67 (150$) or a P67 board with a single PCI 2.0 slot (120$)
Asus DirectCu 6870 (220$)
Ripjaw 8gb (85$)
Samsung F3 1TB (60$)
A case of your choice
735$ - w/o case I may have done my math wrong though.
The GX series is something I would not recommend, as they don't have a very high temperature rating and can actually shut down due to OTP at high loads at high temperatures (above 35C). Thankfully the case you are looking at, the M59 mounts the PSU at the bottom, so hopefully it shouldn't have to deal with heat from the rest of the system.