Problems with my FX 5900 XT

I know this is long but I've been in a 2 month battle with my video card, please read.

About 6 months ago I built my computer and I got a GeForce FX 5900 XT for the video because my mobo didn't have onboard video. So I get my system put together, everything is fine. About 4 months later, I start up the system and I get what can be explained as black lines through the text that says:
XFX GeForce FX 5900 XT
Serial Numbers
Driver Numers

which happens at boot up. Then after that came up with like black lines through the white text, the screen would just be black from there on out. So I call up XFX (after 2 weeks of trying to contact them) and they tell me to send the card back they will replace it. They send it back to me, same exact problem. I send it back to them again, and they send it back to me again saying nothing is wrong with it. So I get it the second time and enclose a letter saying I want a brand new video card (I marked the serial number sticker on the card so I knew it wasn't them trying to send me back the same). So then yesterday I finally get my card back (which I sent to them three weeks prior) and I put in the videocard but forgot to plug in the molex power connector into the back of the card. I plugged the big power cable into the back of my powersupply and then plugged the other end into my power strip, which was on at the time, and I saw a spark, well not a spark but a blue electric current..you know what I mean, between the two but just plugged it in and started it up. The same thing happend, I had lines through the vid card boot-up screen and then black. So I turn off the power and then connect the molex power connector to the back of the vid card and then turn it on again. This time I had like 5 letters of "Geforce FX 5900 XT" were in orange and I thought that was weird, but then it continued to boot up and got to the windows loading screen and that had what looked like spraypaint tool sprayed vertical lines through the loading screen. Once again it continued to boot up and I got to the logon screen which had black horizontal lines through it and I logged into my login name and everything was fine. One I was logged on there were no more lines in windows. Then about 5 minutes after I am up and running the screen goes black. What is the deal with my computer?

One of the fifty-some times I called XFX they told me to go out and buy a new powersupply because appearently they didn't think mine was sufficient (Utra 500W PS w/ 2 fans). The one they reccomended I buy was like $100 so I just called the company back the next day and told them that I had bought the powersupply, although I hadn't...but I was so positive it wasn't the powersupply that is why, and that's when I sent back my second video card. Before I sent it back though I wanted to make sure it was the videocard so I took my computer up to CompUSA and had them test another AGP card in it. The one they tested was not as powerful as mine and didn't need a molex power connector in the back. When they tried the video card it worked fine and my system booted up perfectly. So I am almost positive that it is not my motherboard or anything. I think it is between the videocard, power supply, and me accidently plugging things in in the wrong order. What do you suggest?