If you want to save some cash I suggest buying from Sky2k.com (
R4DS - Sky2k - The nation's leading video game and flash memory) and buying the microSD from eBay or Newegg.com for no more than $10 after shipping for 1GB or $13 with all the adapters. Believe me, I checked all the R4 resellers and Sky2k is clearly the cheapest. You might want to go with the Asian version since it's the cheapest and the English firmware can be patched to work with it.
With 1GB you can probably hold nearly all the homebrew ever made for DS or about 6-10 hours of video in DPG format. Other things such as images and audio don't need to be converted, so it shouldn't be hard to figure out how much of that you can fit.