The SB51 is almost identical to the SS51. The differences:

1. The SB51 will support Hyperthreading when P4 processors start using it
2. The SB51 is a smidge faster in memory benchmarks. Enough to notice, but not enough to be concerned with.
3. The SB51 power supply uses the proper P4 power supply with a 4 pin square power connector in addition to the ATX connector. The SS51 uses a 4 pin inline for the extra power.
4. Faceplate color. SS51 = light blue, SB51=light brown

Beyond that, they are basicially the same. When the SB51 is released, expect it's price to be the same as the SS51 (online vendors are trying to cash in on preorders for them).