first of all, here are a couple links to help you understand amd architecture. first, all a64's run at a 200 HTT (fsb). http://forums.amd.com/index.php?showtopic=22531
pc3200 is the name given to ddr 400. it has a max theoretical bandwidth of 3.2gB/s at 400mhz (since it's ddr this means 2x200mhz)
since most a64 chips have built in dual channel memory controllers(as apposed to a memory controller on the NB like intel has) the max theoretical bandwidth in a dual channel configuration is 6.4 gB/s at 200mhz HTThttp://forums.amd.com/index.php?showtopic=13449
ddr2 ram has higher bandwidth, but has much slower timings.
couple things, though, ddr2 is starting to come with lower timings, but ddr is starting to come with higher bandwidth. ddr2 and ddr require completely different mobos/systems. p4's are using ddr2, amd uses ddr but are possibly switching to ddr2 in the future or switching to ddr500 for the internal HTT. meaning that the internal HTT would be running at 250mhz as apposed to 200mhz.
there are lots of links in the tech guides and tips section of this forum that will help you understand it better