I would get the 6850 or higher because it will handle 1080 at med to low. I would get a 500w PSU one with SLI/CrossFire ready. The brands you should look at are Corsair, Antec, Seasonic, XFX, Silver Stone, and some OCZ brands.
I will link you to a list of good PSU's to the worst so you can maybe match one up within your budget. And finally the MOBO the P61 has only one PCIe x 16 slot. Plus it has only room for two sticks of RAM.
The P67 has 4 slots for RAM and it has two PCIe slots x 16 and x 8 I will link you to both of the ones I found. Just in what I have stated should help you in figuring out how to proceed. Good luck to you.http://www.comx.co.za/PH61-P33B3-MSI-Intel-PH61-P33-B3-H61-Chipset-LGA-1155-Motherboard-Information-Price-Buy-Cheap-p-59739.phphttp://www.comx.co.za/MS-PH67S-C43-MSI-Intel-PH67S-C43-ATX-LGA1155-Intel-H67-Motherboard-Information-Price-Buy-Cheap-p-47199.php
PSU list of good to bad Teir 1 = good teir 5 = worsthttp://www.eggxpert.com/forums/thread/323050.aspx