Re: black screen
XP 64 bit is a bit of a dog, and a lot of companies never made 64 bit drivers for XP.
As for the problem at hand... first thing to check is that you've plugged your video cable into the correct port. There'll be one on the motherboard (in most cases) and one on the video card. Make sure it's connected to the one on the video card.
You've already reseated the graphics card, so the next thing to check is that the RAM is seated correctly. Take it out, and push it firmly into place again ensuring that the plastic clips come down over the edges of the RAM.
Did you have to force the CPU into place or did it gently drop into place by itself? Also make sure the CPU is of a correct orientation compared to the board. (denoted by pin number 1 (probably a little golden arrow on the CPU)
Are both your 24 pin and 4 pin (possibly 8 depending on the board) plugged into the motherboard and seated correctly into their slots?
Athlon II x4 645 || 1TB 7200rpm HDD || EVGA GTX 650Ti OC || 8GB DDR3 RAM || Windows 7 Home x64
i5 4210M || 500GB Samsung EVO 850 SSD || GeForce 825M || 16GB DDR3 RAM || Windows 10 x64