Solid build for your purposes. Your PSU is barely carrying the system, though.
CPU - 95w (120w @ load)
6870 - 108w idle (275w @ load)
Optical - 30w
case fans - 5w per fan (15w for the x3's default 3 fans)
CPU fan - 5w
HDD - 30w
SSd - 3w
478w @ load. after a few years degradation, a 500w 80+ psu won't be able to provide adequate wattage when the machine is pushed. try a 600w 80+.
Another option is the HD 7770 instead of the 6870. Slightly lower performance, but only 142w @ load, and it's $20-30 cheaper than the 6870.