help troubleshooting a computer that wont turn on

i have a computer that i am trying to fix. here is the problem:
when you press the power button, there is no signal to the monitor (nothing at all, no bios, nothing) , and the monitor just goes to sleep . i opened up the pc and saw that the fans were working fine when i turn it on (thinking the powersupply might be ok).
i think it could be two things, the power supply or the motherboard.
my predictions are that it is the video on the motherboard (no video card)

please help me troubleshoot this computer, i have purchased a powersupply tester, but the instructions are not clear at all, so i am no too sure how to use it. here is the power supply tester that i bought.
if someone could give me easy instructions on how to use it that would be great!
Newegg.com - Rexus PST-3 Digital Power Supply Tester with LCD - PC Tools

i would really appreciate your help

A power supply tester will check for voltages, however that is with no load on the power supply and many times a power supply, if it's bad, will not fail until it's put under load (connected to the PC). Thus the testers don't give you the full story.

If it was just the video, the PC would boot up, do you hear the hard drive accessing? if not then there are other problems than the video and it is not powering on correctly.

this particular computer does not have a hard drive light so i cant check if it is accessing the hard drive
is there any other way to check?

luckily, after disconnecting and reconnecting every connection, the computer turned on, but then it turned back off
i then removed one ram stick and that seemed to solve the problem, it has been running for about 5 hours now and i restarted a couple of times and its working fine now
when i removed the ram stick i found out that they were different. one of the sticks is ddr2 pc5300 and the other is pc 4200. i thought this was wierd because those are the stock ram sticks, they were never upgraded or changed. they were also different branded rams
i think that might of been the problem
My next step will be to perform a memory test with the other ram stick using a linux live cd to check if it is bad
i will come back with the results

-edit from my last post--now that it turned on i was able to see the hard drive light