I ran some bandwidth tests with my old PC2-6400 a while ago. I didn't test it in single channel mode, but in dual channel, with memory at 880MHz, 4-4-4-12-2T, Sisoftware Sandra was giving me a total bandwidth of about 8600MB/s. the maximum theoretical bandwidth from a single stick is 6400MB/s, and you are very unlikely to actually get this.
Always go for dual channel, its the only way to go.
@Kennysevenfold: how did you get just 5-6% for single channel vs dual channel?