I have a Powerspec B800 that had a GeForce 8400GS PCIe 256MB card. I wanted to upgrade to a card with HDMI so I can send a feed to my TV. I purchased an EVGA GeForce 210 1024MB DDR3. I installed the card and had video, but once Windows loaded there was no picture...the monitor (VGA or HDMI) was getting a signal (meaning it wouldn't go into powersave mode), but no picture - just a black screen. Checked the EVGA site, which suggested removing a DIMM. Bingo - picture works.
Since I obviously want 4GB of RAM instead of 2GB, I did some research that seemed to indicate the power supply needed to be upgraded. Purchased a new 750Watt supply with (4) 12V rails at 18A each. Installed both DIMMs - same issue, no picture. Remove a DIMM and the video works. The EVGA GeForce 210 does not have a power plug - it gets all its power from the PCIe slot.
I have tried swapping DIMMs and slots, so it isn't an issue with the RAM itself.
I have read that there are some BIOS settings to allocate power to different slots and PCIe, but I can't seem to get anything to work.