opinions vary, but here's what i think....
either go down the cheap route and use and old r4 card or pay a bit more for the ace3ds plus card....
r4 1:1clone
pro's:
real cheap, get them for a couple couple of quid
use woods firmware, so will play almost any ds game
cons:
does not support sdhc cards, so 2 gig non sdhc is biggest sd card u can use
will not work on a dsi
ace 3dsplus:
pro's:
reasonably priced , but a bit more expensive (specially if it just for a dslite)
uses woods firmware so will play most games
card is upgradeable
can be used with dslite, 3ds, dsi
supports sdhc, so can use any size sd card
cons:
cant think of any
there are other cards, but thats what i would suggest