Is the Phenom II 955 a C3 or C2 stepping? You're better off with the C3. Its not a horrible buy no, I personally would try to talk him into $250 for it though, just to see how low you can get him lol, yea I know thats cheap!
But assuming we're talking an AM3/AM3+ board, strapping a 7850 on it would work out pretty well.
Just to give you reference, a Phenom II 965 build today with a 7850, 8 gigs of DDR3 on a decent AM3 board would set you back about $700-$800. With Windows7 though, figure in another $100. Thats why I'd try to talk him down on the price, since you're paying him 400 and going to be putting another $250 into it and its a used system. Maybe I'm just too much of a miser lol.