Well, now it works. I cleared CMOS on the RIIE, loaded setup defaults - all fine. Re-set everything in bios, still fine. Card has been running milkyway for the past 5 hours or so...
Gonna put the waterblock back on tomorrow. Maybe it was an issue with the block, but I doubt it. Pads showed perfect contact on all VRMs, cores, rams and whatnot. Probably just some VERY annoying bios up on behalf of Asus