Welcome to Tom's Forum!
Most 'DDR' MOBO's are 2xDIMM and the maximum is 1GB/DIMM or 2X1GB maximum; 1GB per stick of RAM max.
If you post the exact MOBO model I'd be happy to double-check. If you don't know then use CPU-z and look at the Mainboard tab -> http://www.cpuid.com/softwares/cpu-z.html
First, jaquith is "da' MAN!!!" on Tom's when it comes to memory config/issues. Listen to what he tells you and you'll likely find a workable solution.
Second, make sure your BIOS has the memory options set to default. You want no custom settings in place until you can stabilize the system. A simple reset of the BIOS (pull the battery method) should set you back to the original config if you can't select in the BIOS itself for some reason.