Motherboard compatibility is still possibly an issue, even if it has a PCIe slot. I have an Asus A8N-VM CSM motherboard that has a PCIe 1.0 slot, that I had originally planned to put a 5850 in. I got the card, but could not get it to show any video, even though I had a sufficient power supply and the most recent BIOS updates. I almost RMA'd it, but bought another motherboard (Intel D975XBXLKR, 3x PCIe and cheap!) instead, which worked fine.
Same here. I have an old Epox nForce4 SLI board that I was hoping to put two 5830's in. Like you I got no video when doing this. Common problem with a lot of the older PCI-E 1.0/1.1 motherboards. Somewhat of a bummer as it could be a perfectly good mining machine.
Now with all that said some have reported getting a new bios from their MB manufacturer that solved things. Unfortunately for me Epox has been out of business for many years.