Gotcha. There will be a "surge" at power up, but running a game or benchmark will be the real test of a PSU`s ability to supply the power that it claims.
The thing is, now I have removed the GTX460, and stuck the HD4850 back in, the machine is stable. I can play games and run benchmarks OK, the GTX460 might draw a bit more power than the ATI card, but the difference is not huge, and certainly shouldn`t cause any problem for my 700W PSU (50amp 12v rail).
I`ve got the Corsair PSU, cheap Gigabyte MB and a Phenom II 250 CPU on the way. In other words, I should be able to pin the problem down with a little more component swapping. My money is on the MB being at fault. As I said earlier, the GTX460 has had a bit of a "thrashing" in a couple of other PCs (with the same drivers that I`ve used), and it didn`t lock up once. I`m thankful that the fault does appear often, rather than being intermittent, otherwise it would be a devil to sort out.