OK, here's what I've done thus far.

I was having intermittent problems with crashes, and recently they've gotten really bad.  I traced it down to bad memory, but I thought it was odd that both my slots 0 and 1 would give at the same time, although that's what the memory diagnostic told me.  I tested a new chip in slot 2 three times, and it was fine, so we're okay there.

However, I'm still having trouble getting Direct3D to initialize on two separate installations of Windows.  I've used three different driver versions for my Radeon, and each time I try to test it using dxdiag (go to Display->Test Direct3D), it crashes to a blue screen, no stop error.

So -- how do I determine if it's my video card or a motherboard?  I'd rather replace the motherboard first, because if it's cheaper, but I'd rather not have to reinstall Windows just to find out that it's my Radeon.

Any suggestions?

Edit: I've determined that the memory was bad, but I'm still crashing dxdiag when I test the 3D interfaces.  I'm going to buy a replacement card and see if that fixes it.

Update: I replaced the video card and it's working through dxdiag well.  I'm still getting weird glitches, though, and a few crashes; I'm going to replace the mainboard and CPU and RAM, and test that through both cards.

what I do to pinpoint parts is just swap them out 1 by one with other parts. Like if I have a proc error, it could be the motherboard or the proc. So I just put the proc in my moms pc or my moms proc in my pc. If her proc woks in mine its my proc, if my proc works in hers then my motherboard is bad. I do the same with ram and video.