In your manual (page 16):
Due to chipset limitation, read the following guidelines before installing the memory in Dual Channel mode.
1. Dual Channel mode cannot be enabled if only one DDR2 memory module is installed.
2. When enabling Dual Channel mode with two memory modules, it is recommended that
memory of the same capacity, brand, speed, and chips be used and installed according to the
configurations in the first table for optimum performance.
3. Because of chipset limitations, do not populate both DIMM sockets of the same channel
(e.g. DDRII1 and DDRII2) with double-sided memory modules to prevent system's failure to
start or incorrect detection of memory modules.
Also note the DS vs SS placement. The trouble is figuring out which if any is DS or if SS/DS mix is the problem. So look at the figures for RAM placements.
Try the following:
CPU: DIMM 1 | DIMM 2 | DIMM 3 | DIMM 4
CPU: 1 GB | 1 GB | 2 GB | blank