As I've referred to this thread on another, I thought I'd update you with the results.
The problem turned out to be a design snag within the Abit KT7 motherboard. Somehow or other, its AGP architecture won't interface properly with some graphics cards. My GeForce2 GTS was only occasionally problemmatic; when I replaced it with a GeForce 4 MX440, the computer wouldn't start at all!
Neither Abit nor anyone else has found a definitive cause, but the only real cure is to go back to a PCI graphics card. Incidentally, the power consumption tests I carried out suggested that 145 watts was sufficient, if marginal - as I originally suspected. The 300w power supply that system now runs on made no difference at all.
My system is now on a Gigabyte K7 Triton 400 mobo, running an Athlon 2000+ XP and the aforementioned GeForce 4 MX440, 512 mb DDR ram and Win XP. It works beautifully!