The BIOS actually gives some fine-tuning control. I manually set all the DRAM timings as follows (doing this from memory so I may've gotten the settings off):

Memory Type: DDR2 800Mhz
Tcl: 5
Trcd: 5
Trp: 5
Tras: 6
Trc: 16
Trwt: 5
1T/2T Memory Timing: Auto

Rebooted fine, watched a Youtube video with no crash, plus Thunderbird is no longer complaining. So far so good, I guess the real test will be running it for a few days and trying things out with VMware.

Thanks for the help lanxxx.