Ive had the same problem some time ago..
Sean told me to reseat the memories and it worked.. once.
After that it would only randomly turn on..
Then i sent it to where i bought the computer they told me the computer was fine that it never had any trouble with them turning on..and now i have it back hum... and it just works again.. dunno what the hell hapenned...
I just know i tried different memories i tried a lot of stuff..but it would just turn on (i mean, booting because it turned on but no signal on the screen and 0.4 e the post) when it wanted to.
My specs (to see if we have anything in common):
AMD 64 3500+ (clawhammer :/ i wish it was venice)
nVidia 6800GT
Twinmos memory 1GB
Abit FAtal1ty AN8 Ultra