DDR...SDR? - what the hell, there's no difference...DDR doesn't provide more RAM or something...they're equally insufficient if you ask me...if someone believe 256MB SDR is insufficient, they better say the same for 256MB DDR - or their reasoning has gone awry....but anyway.
i personally would NOT go 256...i've noticed a good improvement/difference from 256 to 384 (or 320 even), and i'd rather have a SINGLE dimm - i.e. one 512MB dimm, as opposed to multiple dimms constituting 512MB...so if you're building a new computer, start off w/ a single 512MB dimm..you'll probably never need to upgrade (at least for a good while).
a single dimm affords greater signal integrity and thus greater stability than two dimms...
and if u're multitasking, u'll notice a difference having 512mb vs. 256mb, even if you're just doing general tasks like emailing, web browsing, winamp, word, etc...(though u probably won't notice a difference from 384MB to 512mb..but like i said, i'd rather have one dimm than two.)