Argh.
I spoke too soon. everything fine for the first two boot ups, and the the BSOD and power error returned.
Had already upgraded to SP2, and have tried having the hard drives on separate 12V cables.
Corresponding with Western Digital now - I think the only options left are either there's a problem with the hard drive itself, or that the Asus A7V333 motherboard is irredeemably flaky (which is quite a reasonable possibility, as it never liked having two different manufacturers memory in the first 2 Dimm slots).
I think we can remove the PSU from the equation, at least. Any other thoughts, anyone ?