I know many ppl here try to put a strong gfx card with an old cheap MB setup to save on cost, well i tried it too with my old 8 year old machine.
I tried attaching a new ATI Sapphire 5870 to a gigabyte GA-K8NF-9 MB and it doesn't work !
I updated the bios to the latest revision (F12K).
I had a 635W GYRO PSU on my old rig (which didn't work with the card) so I changed the PSU to a PSU that was verified to work with the card a new MB (i checked it myself) - the new PSU was a Seasonic 500W.
All these things didn't help, each time i turned on the machine, the fans started up and the HD did a spin-up but the screen didn't turn on and the HD didn't continue loading the operating system.
I will probably need to invest in a new MB+CPU+MEM too (as the old rig had an AMD 3000 on it)...
At least this post will help others not to try the combination