I have the r4ds - it uses the same chipset as the m3 ds simply - and apparently theres a converter so they can use each other's firmware.
Picked up a 2GB micro sd card - still got more than half the space

MUST HAVE HOMEBREW!!!
1.
Quake DS - it's the new help world. NB : /id1 dir MUST be in root dir, place wad in there. Some slowdowns, but remains playable.
2.
DSOrganize - A PDA in your pocket. Browser is text based so it does not need the extra RAM that the slot 2 carts (the gba slot) can add. Includes an IRC client, as well.
3.
Dscent - Descent, controlled via touchscreen. Takes some work to get working, if you have the dos version like i did, that will not work. there's a win95 version (i swiped it of emule) floating around coming with descent 1 and 2. THOSE files work.
4.
Warcraft tower defence - maybe the most awesome homebrew game i've tried. Map editor, FEELS like warcraft 3. Beautifully done. /maps must be in root of sd card though.
5.
Comic book DS - can batch convert your .cbr and .cbz files to cbds format it uses automatically, and is a very polished app. touch screen moves the viewport around, and you see what you're looking at on the top. dynamic zoom. Highly recommended.
6.
DS Linux - Linux on the DS!!! There really isnt a whole lot of apps for it yet, but some key ones such as a text editor are there.
7.
ds aim easy to use and working just fine with aim! nicely done app.
Extra info :
1. you need a commercial rom to setup wifi. The app
dswifi is neat, but just didn't work as well as say, mariokart. wardriving option is kinda nice though.
2. roms you get MAY need to be patched with DLDI. Ok, here's the deal. Lotta games out there used different types of flash, with different chips. The way these roms read and wrote to chips were all different. DLDI is a library that can be used for roms to read and write using standard FAT on flash.
3. Slot 1 is the DS slot. Slot 2 is the GBA slot. While the DS slot will run stuff in ds mode, The gba slot can be used for other purposes - e.g. Opera browser uses the slot to expand memory (think of it as a DIMM!). Also, certain chips are just too big for the ds carts. e.g. the Play yan will run on the gb advance, but not on the ds. It will play back mp4 files quite nicely. Something the 3/66mhz arm procs in the ds cannot do, because it contains an mpeg4 decoder chip in the cart.
4. snes emulation so far for me has been pretty poor, but don't lose hope

nintendo is fine, genesis should be ok too.
And please - if you like a game, buy it. You're already getting the big portability advantage. I'd like to see good creators getting fund to make another good game

couple other links for use
not just a compaability list, but a good list of what's there.
http://wiki.scorpei.com/index.php/R4_compatibility_listand here
http://nintendo-ds.dcemu.co.uk/ you need to dig a little - scroll down looking at the right menus till you get to applications for the ds.