Even with the 1Gb + 2Gb configuration you will get full dual channel memory up till the 1+1Gb limit, and single channel performance after that, so 1+2 is better than 1+1.
See this link for what Intel calls "Flex mode" DRAM.
Quoting the above link:
The figure below shows a flex mode
configuration using two DIMMs. The
operation is as follows:
The 512 MB DIMM in the Channel A, DIMM
0 socket and the lower 512 MB of the
DIMM in the Channel B, DIMM 0 socket
operate together in dual channel mode.
The remaining (upper) 512 MB of the
DIMM in Channel B operates in single
So for 512MB + 1GB configuration, the bottom 512MB of both DIMMs will be in dual channel mode and the top 512MB of the 1GB DIMM will be in single channel mode.
I'm not familiar with AMD motherboards, but they almost certainly have the same behavior.