Just a note, don't necessarily rule out the Power Supply being bad. I had a system that did pretty much what you describe (brief bit of lights, etc., from the drives, no post, no boot), hooked up another PS, and all was working again.
Another thing to try that I've seen work for a similar problem is to completely clear the CMOS memory associated with the BIOS (remove the battery and temporarily short the battery connections, replace battery; additionally, quite a few motherboards have a cmos clear jumper position, which you'll see somewhere near the battery, jumper that for a minute and undo).
I consider these low probability solutions, especially considering what has already been suggested (and you have tried), but I have seen both of these methods fix a couple of my PCs that were exhibiting symptoms similar to yours.
Good Luck!
