Depending on the M'Board in this unit
It may just be the wrong type of RAM being used as well. With many Dual Chanel M'Boards they can be picky when it comes to using more than 2 RAM Modules. I've seen cases when the system refuses to display anything when turned on with 4 RAM Modules Fitted, to freezing after the POST to finally just freezing when the system is loaded.
What causes this to happen is that the RAM is incorrect for the M'Board and while 2 or 3 Modules will work the unit happily when you add 4 you need to go to faster RAM than what you currently had fitted. 
Col
I've seen that happen with high-end motherboards ..
Yep I've seen that happen as well
But not so much on High End M'Boards things like ABit AW8, Gigabyte EP31-DS3L, EP43-DS3L and EP45-DS3L M'Boards will do the same thing.
I have a EP45-DS3L here that will run happily with 2 X 2 GIG 667 Modules Fitted but with the 3 Fitted it's unstable and falls over or hangs randomly. Then when a fourth and it locks up.
Replace the 667 RAM with faster stuff and it works OK.
Col
OM I've been using 1066 RAM on the 43 and 45 Chip Set M'Boards
With all Sockets Populated without a problem.
If that is of any help to you.
OH and I'm not going to answer your question either now. 
Col
OK some easy explanations here
The 391 MEG of RAM is to do with the On Board Video. Some System RAM is stoled from the system to generate the Video. So you loose whatever is set aside for Video in BIOS.
I've had several systems fall over after years of service and in each case it has all been related to the RAM being used. Just recently I have had 3 systems fail to start when they have been turned off for a couple of days. In each case it was a Faulty RAM Module. I just heard a endless series of Beeps from the On Board Speaker till the faulty RAM Module was removed. I even had one new one do the same thing that's the RAM Module not a computer. It was just a faulty Module from new.
It's also possible that as things get old and degrade or the Specifications drift that the unit will run into Incompatibility's but this is more to do with the Specifications drifting and no longer working correctly together.
As for the new PS making things worse what is it and what happens when you refit the old one? At a guess here I would be questioning the quality of the new Power Supply and defiantly questioning just how stable it is.
What happens when you remove 1 RAM Module? Is the system stable now?
As for testing the thing download a copy of the Ultimate Boot CD from here it's free and boot off the CD that you make from the Image and run a full spread of Diagnostics.
http://www.ultimatebootcd.com/download.html
I personally always use the ISO Image as they are harder to tamper with so it's less likely to be infected but none the less you should always check the Check Sums before doing anything with a Downloaded file.
Col
What would cause no boot when 4 mem slots filled but not 3?
ram: 4x512 DDR400
System boots with 3 mem slots occupied, but not with all 4 (total freeze before XP splash screen)
Individual sticks test OK with Memtest86+.
Also, system will occasionally freeze after several hours with 3 sticks installed. Board temps show normal in BIOS. Can't do any tweaking with this bios.
Suspect PS issues. Maybe?
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