I would make sure everything is plugged in first, I know it might sound dumb or something in away but I thought my mobo was fried when I was just plugging the power button into the wrong jumper. So make sure the CPU 4pin power cable is plugged in, and the two graphics card 6pin (i think the 4870 has two) power cables are in and check out this list too http://www.tomshardware.com/forum/261145-31-perform-steps-posting-boot-video-problems
I hope that helps.
Thanks, didn't notice that list because I was looking in the wrong forum. It was a problem with the RAM slots (which is weird, I used the same slot as I did in Thailand).