ga-7vax and ram module

i have the ga-7vax mainboard.
upto now it was running fine with a 256 Mb DDR, PC2100 module.
(in first ram-slot, others empty)

to lift the system a bit, I changed this module for a
1 Gb DDR 400 Mhz PC3200 module..

now Win98se starts and runs, another harddisk with Suse9.3 does so too (AND is faster), yet another harddisk with FedoraCore5 *stops* in the boot process and won't run.

Live CD's with Knoppix do boot and run;
Live CD's with Kubuntu and one with Ubuntu are stopping in the boot process and will not run.

Before (with the old memory module) ALL were running without problems.

I tested the memory module with Memtest, no problems (after 10 passes).

does anyone know something useful??