How many memory sockets. If three, check which of the two slots is for Dual Channel, and try the single stick in the third socket. [just a try]

What memory settings are you using in the BIOS, what CL is your memory, 2, 2.5, or 3? Are the two I Gb.s both the same MFR?? Some mobo's are very finicky about what brands of memory are used [it doesn't know the brands], just that some memory brands timing is more critical than others. With your two prior sticks you weren't putting any stress on the memory timing.

Also some early mobos, supposedly supporting 400 MHz FSB with PC3200, were found wanting.

The mobo mfr site should show the memory brands that they have qualified for that mobo.