So, the project was originally quite simple. Upgrade the AMD Athlon processor to a Phenom processor.
The Gigabyte motherboard supported the AM2 socket system, therefore replacement of the processor by simple drop in and out. Oh, but computers are never that easy.
After failing to boot with constant long beeps: I decided to purchase an ASUS motherboard. After upgrading the BIOS (see above), the machine seemed to boot OK. Certainly boot and stay stable enough for timing tests of Athlon vs. Phenom in a previous post.
Adding the extra 2Gb of RAM just pushed the poor system over the edge. Random reboots and general instability. My first thought was to blame the Phenom. How wrong I was.
Thanks to this comment from Tim, I hunted down the documentation of the ASUS motherboard and found that the Corsair memory I was using was not certified! 5-5-5-12 vs 4-4-4-12 timing memory is not a small thing. Also, there are voltage differences between the memory sticks.
So CM2X1024-6400 did not work, but the CM2X1024-6400C4 is working perfectly well – 4Gb installed on a Vista x64 24×7 and under CPU load.
Lesson learnt: read the supported memory documentation!
Oh, and the Phenom is a good 20% or so better transcoding video. Phew.