The only difference to my knowledge between the 9650 and the 9550 is 170MHz. I see on Newegg that you're looking at about $325 for the 9550 and $550 for the 9650. I'd have a real tough time convincing myself to pay approx 66% more for such a small increase, unless the 9650 somehow overlocks much better. I'd probably go for the Q9550 as I doubt i'd ever see the difference except for a few extra points in benchmarks.
As for the 4870X2, two of those would be beastly to say the least, but I would say SEVERE overkill unless you're bound and determined to play bleeding-edge games as high as you possibly can and beat all your friends' 3DMark scores. I would probably start with just one. You can always add another later if you so desire. And you'd need a really beefy PSU to run two of those.
I say get one 4870 and if you aren't getting the FPS you want add an additional one. The difference in processors is the multiplier. The Q9450 has 8x, the Q9550 has 8.5x, and Q9650 has 9x. That's it. They all have the same manufacturing process and voltage specifications.