The nForce2 has a twin bank memory architecture, so if you want to take advantage of that, you need to populate two of the DIMM sockets. You don't have to though. I believe it will run fine with just one stick of 512 memory installed, you just will revert to the 64 bit mode.
As for being better, yes most reviews conclude it is. The nForce boards are beating the KT400 boards in most of the benchmarks and the nForce seems to be getting high ratings for stability.
700 Mflops in SETI!