Video Card Upgrades on Dell e520

I’m attempting to upgrade video on a few Dell Dimension e520 PCs. I’m using Sapphire x1550 (ATI) and I end up with no video on boot. I’ve checked the BIOS settings and it is set to detect a card in the PCI-E slot and override the onboard video. No problems there. So far none of these cards seem to be working, and I’ve tested several to ensure I didn’t just get a bad one. I even went so far as to replace the 305 Watt PSU with a 500 Watt model to make sure that the problem wasn’t for lack of power.

Any ideas?