Weird reboot problem

My son has an HP A1210n computer I bought for him last year. It came with a 2.2ghz athlon64, 1x512mb ram, onboard video, 200gb hd and a lightscribe dvd drive.

Since then we added 2x256 ram I had lying around and a 7600gs video card.

Lately there have been rebooting problems as in the bios screen not even starting.

When the computer is running, it is usually fine, but if you need to reboot it usually won't restart until it is left sitting for an hour or so.

It almost sounds like a heating problem although all the fans are working.
It is running stock cpu cooler and motherboard chipset just has a heatsink. There is one case fan at the rear, blowing out.
The cpu and gpu are not oc'ed.

I am also thinking it could be a power supply problem. I looked at the label and in small print at the bottom it said - 300w max. It is the stock generic psu HP uses for thier low end computers.
Or could it possibly be a motherboard problem ?

Not sure which direction to take first.
Program to moniter temps ? (any suggestions which one? ) also what is to hot for this cpu ?
Replace psu ? or possibly motherboard...

Sorry, so many questions. I have tried to research and narrow the field but didn't have much success.

I thought if someone else has encountered this they could share thier experience.

Thanks in advance.

Thanks for the reply, I did not give the ram much consideration.
We are going to try variations on the ram as well as unpluging the dvd drive to help free up some power load. Maybe even try the onboard video again just to check.
Looking at the psu label again, it showed +12v 19A and had a small disclaimer below saying '+5v & +12v shall not exceed 268w'

It looks like the video card and other components are pushing the limits of this psu.

On one site I looked at was showing the 7600gs requiring a 300w psu.

We removed the 2 256m ram chips and it booted fine this time. I think we will get a new psu reguardless. In the mean time I am searching for that memtest program.
I am thinking there may be multiple issues here.
The booting problem is so intermitent it is not a quick one to track down.

Thanks for your suggestions.

Ok Thanks !
Nero will take care of the iso

I have memtest built into the bios on my mb but other than that, I havn't used a cd version.
I will burn the iso and pass it on if I can get him to park his Dark Messiah game for awhile

Thanks for your help.