I think the R4 cards are supposed to be the most compatible - just make sure you buy a real one and not a dodgy one.
I had mine bought for me, so not sure the best place to buy them.
The one I got had the blank cart, a small memory chip (2gig I think) & a usb stick to put the mem card in so I could transfer the downloaded files from the pc to the usb stick & then take the mem card out & plug into the cart - which then went into the DS
You can fit as many games as you have space, but I think 2gig is the max for compatibility issues. Games range from 8mb to 64mb generally.
Bookmarks