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.
I have 2 512mb chips in my DDR 400Mhz Mobo,
One is PC2700 and the other PC3200.
I Just bought a 1g stick of PC3200 and replaced the other chips with it.
Now the computer works fine, including running BurnInTest Pro until I run a game, doesnt matter which game, but it crashes, just resets.
I have swapped the stick with another 1g PC3200 and it does the same?
Anyone got an idea?