Upgrading to the i7 would require a new motherboard with support for DDR3. The difference between that and the q9550 is mainly that the i7 is a newer architecture and has 4 more threads in addition to the 4 cores. (virtually making it an octocore)

Personally I wouldn't upgrade XP to XP 64 bit, but only because I've heard some pretty bad things about it. I haven't tried it myself though, so I really don't know just how bad (or good) it is.

Out of the three graphics cards you mention I think the 7900GS would be the best, but truth be told, neither of them are that great.

Since upgrading your motherboard is too much money for you right now I'd choose the q9550 and then later upgrade the motherboard and get more RAM. And in all honesty the q9550 isn't that far away in performance from the i7 in gaming.

Hope I could help you out.