I can't exactly say why it isn't detecting your new video card, but I can say this:
I ran the Radeon 5570 on a 250 Watt PSU for a long time. (6 months+) so it isn't that. The wattage is really only recommended when you're doing GPU intensive tasks. Granted, you want to be around minimum, but for daily use, it isn't completely necessary. If your PC isn't detecting your video card, it may not actually be "plugged in" to anything. If it takes no external power supply, does it plug directly in to the Motherboard? What card is it, by the way?
I know I'm not too good with GPUs yet, but I hope this generally helps you. I had a similar problem when I first got my card, too. If your PC isn't too old, your PCI-e slot shouldn't be bad. Does your motherboard accept external cards?
And what are you using? HDMI, DVI, VGA?