Sounds like you're covering the basics at least, and if you're comfortable taking it apart and putting it back together, that'll make it easier on us.
Do you have another video card (or onboard video?) that you can test with. Sometimes when a video card's BIOS can't get through POST, it'll sit there spinning forever.
I'd also reset the CMOS jumper on the board and see if that helps. Coincidentally, it'll also re-enable the onboard video (if any) and allow you to boot to the onboard video, if any.
Failing that, I'd try another power supply. If no go, motherboard is my final guess.