I have an i7 (3770K@4.2GHz – so not running at the highest possible stable OC which is around 4.7GHz), 16GB RAM with an AMD 7870 (HIS 7870 IceQ X Turbo X), running the game at 1920×1080, all options on highest possible setting and my fps doesn’t drop below 60 in PvE at all. Neither in SPvP. The only time I see drops is in WvW.
As my slightly oc’d 7870 is as fast as a 7950 in benchmarks, you shouldn’t have any problems when going with the 7950. I mean, hey, that card also plays BF3 on ultra@1080 with no problems, so GW2 shouldn’t give you a headache.