A 6870 is like the 3rd best card... Oh yha you'll be good... The Antec psu will probably out live everything. Thats the beauty of a quality peice. Expect 5 or more yrs no problem... Unlike parts like cpu/gpu/ram psus don't get faster or slower, they just go boom.
1 thing though... Mesasure your case where the gpu sits.. Theses type of cards are longer than the basic cards. This ones 250(L)x115(W)x40(H) mm Size
Btw, never noticed, but it may come with 2 free games, dirt3 and Deus EX.
There, sell them for 15 quid each and your back to your 170!