You must have same size config., even if you choose to put three sticks in there. Get all 512MB or all 1GB sticks. That 512MB at the end, move that up. I think it should work.
For three-DIMM dual channel mode, the DIMMs in Channel A must match and the DIMM in Channel B (either slot) must be equal in capacity to all of Channel A.
Rules to Enable and achieve Dual Channel mode, the following conditions must be met:
Matched DIMM configuration in each channel
Same Density (128MB, 256MB, 512MB, etc.)
Matched in both Channel A and Channel B memory channels
Populate symmetrical memory slots (Slot 0 or Slot 1)
Configurations that do not match the above conditions will revert to Single Channel mode.
The following conditions do not need to be met:
Same timing specifications
Same DDR speed
Memory channel speed is determined by the slowest DIMM module populated in the system.