this is gonna get a little weird so let me start at the beginning.
i found an old crappy gateway in my dad's basement, so i had the bright idea of turning it into an HTPC. to do this, i bought a powercolor radeon 5450 so that id have an hdmi out, and i completely wiped the drive and put ubuntu on it.
when i plugged in the card, i got nothing. just a black screen and some loud fans. this happens through both the vga output and the hdmi. pull out the card, and its fine, works like a charm. I then put the card into my other pc, and it works fine in there, so the card itself is not bad.
Now i know what you're thinking, it's probably the power supply. That's what i thought too, so i upgraded to a corsair 600w. Well I'm still having the exact same problem.
If anyone could help me out it would be greatly appreciated.
It's possible that your new graphics card is PCI-E 2.1 and your the motherboard on the old system is PCI-E 1.0. If this is the case, check Gateway's web site to see if there is a BIOS update for the old system that SPECIFICALLY states PCI-E 2.1 compatibility.
If there is a BIOS update for this, you may want to try it. Otherwise, I'd just look at getting an older (PCI-E 2.0) graphics card with HDMI. Think Radeon HD4xxx series or older, or an NVidia card.