BSOD - Vista BCCode 124 - Why?

Just because it happens when your CPU/GFX card is under load doesnt mean its those 2 things. When BSODs happen, your RAM is the first thing to look at.
Download Memtest, burn it to a CD and run the test (to do this you'll need to reboot the pc, go into bios and set the disk drive to boot order 1, so the test runs before windows starts).

Run about 2 or 3 passes, if your RAM throws errors (which it shouldnt) then your RAM as at fault. If not, run chkdsk (google it on how to do it) which will check your Hard Drive for errors.

Run Prime95 Torture test to check for CPU errors (if your CPU is faulty, it wont take long).

Unsure about the 275, if you have access to another card known to work just fine (not necessarily another 275, just anything nvidia) see if it still blue screens with that card in, if it still does then your card is less likely to be the culprit.

That PSU looks to be a little low wattage (even if it runs fine), just a thought..

If you've overclocked anything in your setup, make sure they are clocked down to standard settings during your troubleshooting.

Hope this helps.

Still happening, the problem seems to be getting worse. I really think I've got something heat-related here, as the system will BSOD multiple times in succession after the first if I immediately reboot - if I wait 5-10 minutes, it's fine. However, the Intel Desktop Utilities hardware monitor never raises heat or voltage alerts.

This error 124 is just obtuse - is there absolutely no way to tell which component is failing? Is there better error reporting in Windows 7, so would upgrading to the beta help my situation here?