P4S8X-MX MOBO not working with PC3200 RAM (or not very well)

I am having a tough week at fixing PCs for my friends. I'm at wit's end here.

ROUND 1: I have a PC that got hit by lightning. Replaced PSU, Mobo (enter Asus P4S8X-MX), cpu fan. The PC worked great for about 3-4 months.

ROUND 2: Since I did a good job, I got rewarded by helping install a new dvd burner, a quiter cpu fan, and the issue at hand here....upgrading the RAM from 2 sticks of 256MB PC2700 (333 Mhz) to 2 sticks on Kingston value ram 512 MB PC3200 (KVR400/512R).

All my problems center on the RAM. The problems started intermittently. So it took a few weeks, but got the PC back after round 2. So now I'm on round 3.

ROUND 3: I downloaded MS memory tester and promptly ran the test. It failed on the LRAND tests. Sent PC2700 memory back to Kingston, got 2 new replacement sticks. Ran tests again --- fails again.

So I put in the old 2 sticks (PC2700/333 Mhz)....passes all Check It Diagnostics and MS Memory tester. Put in both new replacement sticks (PC3200/400Mhz/2x512MB).....tests fail.

Put in one of the new 400Mhz....fails. Put it aside. Test the other 400Mhz, passes.

So, I decide Kingston memory sucks and buy 1 GB (1 stick) of PC3200/400Mhz) that per PNY will work with this motherboard. (Mind you Kingston was on the QVL from Asus too but that didn't help).

Well, this stick (Kingston PC3200 1GB), won't let the PC POST. Reset CMOS. Won't POST.

Add the "good" Kingston (512) with the 1 GB (PNY), system works, but fails the memory test.

Take one of the old 256 with the new 1 GB, memory recognized, system boots, fails memory tests.

The only thing I haven't tried is the old 256 with the new 512....I'll try that next.

My questions are:

If MOBO, or CPU are bad....why does the old 2 sticks of 256MB/333 run fine every time I test?

Why won't the new 1GB Kingston POST up?

Thank you for any insight you may have!