Because the HTT that's 1600 or 2000 is because of the hypertransport. It's still shown as 200 in the BIOS, but it's 200 x the HTT (for a 1000MHz board it'd be 5) so 200 x 5 = 1000x2 = 2000, that extra 2 is because of the fact of the 64bit nature of the CPUs.
The RAM works the same way, it's still shown as 200 or whatever it may be, but due to the hypertransport it's got that bigger bus.
Also as it is, the higher frequency RAM sticks have really loose timings so basically they need to develope a new style of RAM chip or something.
I don't know about that DDR333 not being much worse than DDR400, sounds like a load of crap to me, as you'd have a bottleneck in your system.