Dual channel simply means you have a 128 bit memory interface, single channel "only" gives you 64 bit.
Performance difference totally depends on application. Some applications like the extra bandwidth, most don't care.
If the increased load of adding another DIMM to your system drops the memory from PC3200 to PC2700, then in apps that don't care about the extra memory it slows the system down.
I find it interesting that some S754 boards had 3 DIMM sockets, and yet I never saw a S939 board with 6 DIMMs.
ISTR adding the third DIMM on some S754 boards turns off the sleep modes as that DIMM had to go through a buffer chip and the other 2 didn't. Take that with a pinch of salt though
