Existing DDR400 (PC3200) modules would work fine with A64 754 and 939 system.
If getting new memory modules, DDR 500 memory module, or even DDR 550 are preferred, even with higher tRCD and CAS latencies. With the DDR500+ (PC4000+) memory modules, memory bus can be overclocked to the level 250 - 300+ MHz. Memory bus at such 250 - 300+ MHz level delivers significantly higher raw memory bandwidth than that at the low 200 - 230 MHz even with lower latancy of x-2-2-2 by most PC3200 modules. Though some existing programs, drivers may not be able to take full advantage of the high raw bandwidth, have to see whether future programs, OS can benefit such excess bandwidth.
Some low latency modules with DRAM chips such as BH5 (new memory modules with BH5 are hard to get currently (2004)) may be able to overclock to 230-250 MHz with tight timing x-2-2-2 and enough high Vdimm (if choosing to use 3.0 - 3.3 V).
Modules with certain Micron 256Mb DRAM chips (-5B), Hynix chips (BT-D43) seem to overclock very well to the 250 - 275+ MHz level with relative low voltage (2.x V), lessen some concerns about high Vdimm on the CPU memory controller interface. There are inexpensive HyperX PC4000 (SamSung TCCC chips) rated 2.6 V to at least 250 MHz, Samsung TCCD chips (4ns) to 250 MHz level with CAS 2.5. Some A-DATA Vitesta modules can run at high memory bus to 250-300 MHz. OCZ EB PC3700, PC4400 series overclock very well with high bus frequency with low equivalent latency.
There are tradeoffs in overall performance (not just raw memory bandwidth) between low latency x-2-2-2 at 230-250 MHz with 1T (command rate), and higher latency between x-3-3-2.5 and x-4-4-3 at 270-300 MHz with 2T (command rate), test out these two cases for specific applications and benchmarks.
