Photo editing is not very demanding on a computer. So any mid-grade or above computer will be fine. The only thing really to watch is ram. Make sure you max it out if you get 32bit. ( 4 gig) I would at least double that on a 64bit system as 64 can actually be slower than a 32bit system. The only real advantage to going 64bit is the added ram. But it can come at the cost of a slower system.
Onboard video is fine for photos as well. As nothing really moves, a fast frame rate is not needed, and there will be no polygons to shade. Again any mid-grade card will more than sufice. The monitor is by far more important than the video card. If I were to spend money it would be on the monitor.
I got the Gateway GT5658E. It was over kill. But I was thinking of doing some video with it as well.
I am a AMD fan, but they have not been able to keep up. So go Intel definatley.
I say move to Vista. I have experienced nothing bad about it. Its almost 2 years old not. 98% of the problems are worked out.