With the Pentium 4, the 200Mhz is the speed that the FSB runs at, and 17 is the multiplier for the 550 model. The multiplier is either raised or lowered depending on the P4 model.
The Athlon 64 is similar, however they dont have an FSB, they use Hyper Transport Link (HT), so for the A64 it's 200Mhz HT x the multiplier, in this case 11.
As an example, the current Core 2 Duo's would be 266Mhz x 8 = 2128Mhz (2.13Ghz, AKA E6400).