hmmm...i dont know if this is an issue or not...but, is the fan on the heatsink plugged into the CPU fan header? Some boards dont fire up the CPU if it's not...
and...the RAM, when you seated it, you felt the "click", right?
and...the video card is seated well...right?
also...here's another basic-ish question, but it does happen on occasion: you did close the socket latch, yes?
At any rate, from what you've told us...I'm pretty sure the problem is RAM, mobo, or video...do you have any other parts (or know people/stores which do) that you can switch and test?