Powercolor 7950 boost or HIS 7870 Ghz Edition??

Go for the 7870. That HIS cooler is beastly, and will give you a gigantic overclock headroom, you can OC forever but you can only go so far until your ears bleed from fan noise. The 7950 with that cooler will be crap, if you were to overclock them both as far as you could get them no doubt the 7950 would be faster, difference is the 7870 you are looking at will be decent on noise, where as that 7950 will probably be so loud you'll not want to use it like that.