if you think, its even a tiny possibility that you MAY want to run 2 cards down the line, go with Nvida from day one. instead of a 7950 get a 670.
i just sold both my 6970s that i was running in crossfire and i got a single 680. Ill never go back to Radeon. my 680 destroys my CF 6970s... drier support is so much better, the update their drivers a lot sooner when new games come out...
ive had 5 cards, this 680 is my first Nvidia card, before i would hear " oh nvida has better drivers, this and that herp derp " kinda dismissed it.... no man, they were so right! drivers make the world of difference!
go with a 670.
also, i wouldnt get a HDD at all if i were you. if i could do it over, especaiily with the way the pricing have come down on SSDs, id get a SSD only, and everything else you want to store, music, videos what ever, put on a external...