black screen after trying to use new gpuFollow

The 550 TI requires a power supply of at least 550 watts. Make sure you have the auxiliary PCI express power leads plugged into the video card, and make sure your power supply provides adequate wattage. I believe that particular card also requires a pci express 2.0 xompliant slot, though it may be backwards compatable. What specific motherboard are you trying to use.

Can you see anything via the onboard video card with the 550 GTX installed? if you can't see anything over the onboard port with the card in, it's probably a power issue.

Ok, a couple things. For starters, what model power supply do you have? if it's one of the chinese generic knockoff power supplies it may say 450 on the side, but they are quite often under rated. Second, that 450 requirement is the minimum power required. that's assuming you are running a minimum configuration hardware setup, only 1 hard drive, only one optical drive, one stick of ram, the minimum number of case fans, etc. I'd almost be willing to bet money you are under watted at this point.

You may have to reset your bios to get back in at this point. Don't do that unless you have a working knowledge of computer bios and how to set boot order of drives, etc. but if you do have that knowledge, you can reset the bios either by pulling the bios battery for a good minute or so, or by finding the "clr cmos" jumper on your motherboard. Make sure you note which way the battery goes back in if you go that route. If you don't know bios well enough, you may want to have someone help you with that part.

If resetting bios doesn't do the trick, you may possibly have damaged your hardware by trying to boot you system on inadequate power. It's especially stressful on the motherboard capacitors, and biostar isn't exactly known for using quality components on their motherboards. inspect your motherboard for any damged capacitors, any with bulges in the top, or split crosses, or any oozing fluid of any sort.