Im looking to build a new system - it seems that the cpus
on offer from amd only support ddr memory up to 400mhz (even
though they have a 2gig fsb) while the intel chips will
support up to 667 ddr2 memory (even though most intels only
support an 800mhz fsb).
Is the memory speed limited by the cpu arch, motherboards,?
Wont the slow ddr memory (3200) become a choke point
for the system?
Memory speed is limited by both, as well as the memory technology itself.
DDR memory specifications say DDR400 is the fastest DDR. Unofficially, there is DDR that has far exceeded that (500+), and with proper choice of mobo, you can utilize the memory at that speed.
Memory has been a choke point since the 80386 days.
AMD doubled memory bandwidth by going dual channel with socket 939. That doubling of memory bandwidth only increased performance about 5%, so I'd say you're not losing much if anything to that perceived (but not real) 'limitation'. AMD is much more latency-driven than bandwidth-driven, so they don't need DDR2 yet.
DDR2, because of its increased latencies, actually performs slower than DDR at the same speed. It takes DDR2-667 to get better performance than DDR400, and most of that performance increase is lost if the FSB isn't up there too.