Otherwise.. grab a multimeter and stick it in one of the 12V rails, watch the amperage when you boot. If it drops significantly then you have a PSU issue.
More than likely however, is the overclocking, if the system bus is out of clock with the PCIE frequency video will fail to boot. I had a 4890 that worked fine in my X38, but I had to reset RAM timings to get my 5770 to work.
i don't overclock my system and i'm 99% that it's not from my PSU. i will try a 6850 (i see that uses almost the same power as 5870) and will see then. i have tried with only one memory stick and it doesn't work so it shouldn't be from ram. at this point my guess is that it's a faulty card