haven't tried taking out all gfx cards... but I have switched it out to no avail. Do you think that it might be the card slot or something?
when diagnosing computer problems -- the technique is remove and swap everything you possibly can. you want to eliminate possibilities. so if there are cards you can remove, remove 'em!
I prefer starting from an absolutely base system and working forward from there (assuming onboard gfx):
1) CPU + MB = beep codes
2) CPU + MB + RAM = BIOS
3) CPU + MB + RAM + ODD = run a LiveCD
Gfx card would be the last thing I plug in since this system doesn't need it.
The less components you start with the better, AFAIAC, since I never trust what I swap in unless it came from a known working system. It also progressively loads up the PSU, (I know you've already swapped it), who knows, it might be that plugging in the gfx card or a HDD causes an unacceptable voltage fluctuation on the supply rails.
If it fails at just CPU+MB+RAM, there's only really two things I'd try: reseating the CPU and/or reflashing the BIOS
BTW, I know you've said it hangs with either RAM stick but who is to say they both aren't bad?
Unless you can test them in another system, (or plug known working RAM in), you'll never know.
It all comes down to how much time/money you want to spend on it.