PCI Experss is coming within the next few months to the A64's. DDR2 won't come until the middle of next year at the earliest. Look at the benchmarks for DDR2 around the web. It will take DDR2 667 or even DDR2 800 to pull ahead of DDR1 400 and the AMD cpu's are not bandwidth starved but rather are more latency sensitive and DDR2 memory is higher latency (i.e. worthless to AMD cpu's at this point). Also, since the memory controller is integrated onto the A64die it is harder to simply replace. DDR2 is marketing hype at this point from Intel. Don't fall for it.
Look at this difference in performance between dual channel A64's and single channel A64's. Only a fractional difference which suggests that higher frequency ram will not make as big a difference as other architectual improvements such as the new core revision 90nm coming out in October/November time frame.