I got lucky and picked up a DS off freecycle a while ago, one broken hinge but in good working order. I was curious about the DS homebrew scene and bought a R4 Flash card for 4.99 delivered from China. I added a 2gb micro sd for 1.99 too. This R4 doesn't support sdhc cards.
It was very easy to setup, I just copied the Wood R4 firmware to the sd card and was able to boot into the R4 card interface. Its not bad at all for running emulators though the DS isn't exactly quick so it will struggle to play some emulated games for sure !
So far I have a great Spectrum eumulator working as well as a Megadrive, Mastersystem, PC Engine and a SNES. Fairly impressed so far for 7 quid you cant go wrong !