I'm ahead on the score!
So far I've gone up to 981 MHz (processor) / 140 MHz (Front Side Bus) / 35 MHz (PCI Bus) and with one 128 MByte DIMM the system is pretty stable.
I've put all 4 and at the end of the test (moving about 650 MBytes of files from one partition to another partition on the same disk and then deleting them) the system blew into a BSOD.
I've taken 2 of the DIMS (so I stayed with only 2 of them -- 256 MBytes of RAM) and apparently the system remained stable (even at 981 MHz / 140 MHz / 35 MHz)....
I'll need to rune more tests to see how far can I go but so far it is installed and working.