Computer boots to black screen

Specs:
Motherboard - Gigabyte K8NSC-939
Graphics Card - ATI Radeon x800 XL
Old Graphics Card - ATI Sapphiron 9600xt
RAM - 2x 512mb PC-3200
Hard Drive - Western Digital Caviar SE WD2500JB 250gb IDE
Old Hard Drive - 40gb Maxtor IDE
PSU - Allied AL-B500e 500watt

Here's my situation, I just decided to upgrade to a ATI Radeon x800 XL from my Sapphiron 9600xt, and from my 40gb Maxtor HDD, to a 250gb Western Digital HDD. I carefully installed the 2 parts and took out the old HDD and then went to boot my computer. It booted, but the screen stays black and I can't see anything. Not only that, but my keyboard and mouse show no sign of working either. I can hear the fans spinning and the computer sounds like it's working, but I just can't see anything happening on my monitor. It just stays in standby mode.

A wierd but notable thing is that between today and the Xmas (when I got the new RAM, CPU and Motherboard), I have attempted to vaccuum my computer 3 times. The first time I experienced a light version of what I have now. The only thing I removed was the Graphics card to clean the AGP slot. When I went to boot my computer, it showed nothing on the monitor. After plugging in the GFX card two extra times, it worked flawlessly from there. The second time it was the same scenario, but it took me about 1 try. Just recently, the fan on my 9600xt started making a little noise so I took it out to fix it. I wasn't able to fix it, so I put it back in. Low and behold, my screen was black on bootup. It took my till my sixth try, which was my last attempt, to get it to work.

I have had my Engineer father and a computer friend try to help me with the situation. We have tried a couple things to try and fix the situation. First of all, I tried clearing the CMOS three times. I have also disconnected the HD, CD-Rom, sound card and RAM and attempted to boot, but it's always the same exact thing. I also tried my old GFX card once, as well as my old HD. I am completely stumped and have to turn to the internet for some help, as noone I know has any clue how to fix it without buying a completely new computer (which wont happen). If anyone out there has any tips, things to try or ideas, please post. I would appreciate it more than you can imagine. Thanks for reading.