Video Card Help

I got an ATI x700 256MB graphics card. When I tried it on my computer I found out that I needed a larger power source, so I bought a 500w. Now when I plug the new video card in the computer runs but the monitor stays black. I have uninstalled the old video card driver and I believe I have installed the new. (Though I’m not 100% sure, because it ends in the middle of installation) Any Ideas what could be the problem?