Okay. More update. Since the pc in question booted with the 4870 I decided to plug back in the rest of the USB, hard drives lights etc. No power to vid or USB again. (vid card fans are on but they were before too).
Edit update2: I restartes it again with the 4870 and it booted fine. 8800 back in, no go.
So either the psu isn't liking the video cars or the mobo hates it