I got an acekard2... awesome cart. Allows you to play homebrew, backed up games, etc, and has nice features.
At DealExtreme It takes 2 weeks to get, but hey, cheap and great. Plus, the Acekard team has awesome support for their product, so if something goes wrong with it acekard.com support will help you directly. I definitely recommend Acekard 2!!
For a microSD card, definitely 4gb+... you'll want space for your games/music/movies etc. Get it anywhere, but I'd recommend Newegg.com They usually have great prices and even better customer service... seriously, really good.
As for apps:
moonshell (medai player- a must)
bunjalloo 0.72 (web browser)
ColorsDS (awesome drawing program that has this cool playback feature shows you a sped-up video of you making your drawing)
DSNotes - take quick post-it notes and save them on your ds
C|IRC (irc client if you're into that)
DSFTP (you can access an ftp server and download files right to your ds, unfortunately, you have to manually edit the server location in the config file, which is only a problem if you access more than one server)
DSOrganize - well, has a web browser, an irc client, a hombrew database thing that lets you download homebrew from specific databases right to your DS. more.
LMP-NG - looks like/acts like an ipod music player
you'll also need BatchDPG (a windows app) to convert your ripped dvds to dpg format for playing on the ds. Don't worry, the vids only come out to about 300MB, so you can fit quite a few on your microSD.
As far as Games:
well, definitely pick up Stay Alive DS - its a 2d Portal remake with great levels and a level editor so you can make your own. Great game!
Anguna - pretty fun zelda-like "rpg"
World of Sand
GeoWars
Setsuzoku - surprisingly fun!
what else? I dont know - Im still having lots of fun with Stay Alive
well, have fun, and yeah, don't expect too much from web browsing. it rots. you might want to look into getting the official Opera web browser with memory add-on.