so you've taken the cpu off and put thermal on? wow, and you've bought new ram..
With a black screen coming on when you switch the computer on theirs alot to try..
What is your harddrive? is it the old IDE cable? if so, taking the IDE's wires of and changing them can confuse the system resulting in a blackscreen, had that problem along time ago.. only way we fixed it was we switched the IDE around multiple times untill it showed the screen, eventually it fixed it and worked.
Tried taking the CMOS battery out the mobo?
have you got a onbored VGA slot? or are you using a graphics card and the mobo doesn't have an onbored VGA support?
Is their any beep noise when you turn it on and leave it?
Try taking the motherbored out, maybe a screw is under the motherbored stopping the circuits.
Have you actually tried taking it all apart bit by bit, everything out.. try it, it's amazing at how many times this fixes these problems