I recently installed a new Sapphire Radeon HD 6870 into an ASUS M2N-SLI Deluxe and it did not boot. Blank screen. Did not POST. I tried everything. I RMA'd it and tried another card of the same chipset (XFX Radeon HD 6870) and I got the same results.
Someone mentioned that I should try powering it off, then IMMEDIATELY powering it back on while there's still juice flowing in the hardware. I did that AND IT WORKED!!!!
I'm posting this now so that hopefully it'll be easier if somebody else has this problem.
This seems to happen on various older motherboards and/or power supplies, and I have no idea why. Everything worked fine when I put my old graphics card in, but this new one seems to require this funky little bit of finessing to get it to start up.