If it's for that Athlon computer you mentioned in your other thread, you might not want to tie your hands getting 2 256 meg sticks. You'll gain a bit of performance with 2 sticks over 1, but when the time comes to upgrade past 512 meg you'll be pissed off because you'll have to buy 2 whole sticks again. If you get the single 512 meg stick now, you can add another 512 meg stick later and not only go to a full gig of ram, but also take advantage of the dual channel memory controller.
512 meg memory is ok for TODAY, but unless you're a benchmark freak you may not notice the difference between dual channel and single channel DDR. If you think you'll ever add more memory past the initial purchase, get a single stick.
IMHO.