but I'm not sure what flash-SD chip thingy to buy. I'm looking between M3DS and R4DS.
Is either one significantly better than the other? For that matter, what kind of software am I going to need to run the apps I get?
Looking into DS homebrew...
M3DS real. it's updated regularly and supported more than any other device. i use it myself and swear by it. you can grab the most up to date firmware from here.
I second M3. I have the M3 Mini-SD slot-b device which is still supported by them to this day. They've done a good job. Slot-b devices in my opinion are nicer since you get GBA support.
Okay, cool. That strong of a recommendation from two people sounds good by me.