Looking to upgrade my GPU

[Exercitus Mors]

Im looking at either the R9 270x or the HD7870. But right now my psu is only 450w. So Im thinking about getting a 800w. If I get a 800w psu will it fry my motherboard? Also how much wattage does both of them take. My specs:
1. CPU- Intel Core i7 2600 @ 3.40GHz

2. MB-Gateway FX6860

its this same one

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=602037

 

[sosofm]

With 7870 or r9 270X a good brand PSU 550W is enough http://www.guru3d.com/articles_pages/amd_radeon_hd_7850_and_7870_review,7.html
You can have a 1200W PSU it will not fry your motherboard.