I'm sure the more expensive R4 cards may be the "better buy", but if you're okay with not spending $50+, i'd recommend the following...
I have 3 different R4 cards, an "SDHC-3DS" (for all DS's), a "Revolution" (DS and DS Lite only), and a "Revolution: Unlimited upgradeable" (all DS's).
I like them because they're cheap, around $10 and are very easy to use, with simple, click and drag interface and subfolder customizability.
Once you have acquired one, all that is needed is to copy over the latest firmware onto a formatted Micro-SD card, setup the folders the way you want them, I.E., "1 - DS games", "2 - Emulators", "3 - Apps", "4 - Music", "5 - Picture", "6 - Videos", "7 - Ebooks", "8 - Retro Videogames (load from folder 2)", etc...and then click and drag your multimedia files onto the main card memory.
You don't have to set the folders up like that, but that's how i do it just to make it easier for myself and anyone else to pick up use right away.
If you'd like, i can offer the sites to acquire the cards and the software data (they call it the latest "kernel"), and all the knowledge i've learned about them up to this point, wich could prove to be quite time saving for someone new to them.
At the end of the day though, if you're decent with downloading data files, extracting them, creating new sub folders, clicking and dragging files into them and following simple instructions on the emulator's "read me" files, then you will be right at home with these and shouldn't have any real problems interfacing them at all.
After that, it's time to turn on the unit and enjoy the fruits of your labor.
Right now i think i have enough multimedia on one 8gb supported card to last me a lifetime, literally.
I actually can't help but feel a little overwhelmed sometimes.
So many great games, so little time... 3)