R4 full name is Revolution for DS, but it's more commonly known as R4.
The link to the website is --->
http://www.r4ds.com/r4.htmPrice is pretty okay, correct me if I am wrong, I got it for 60+ with a 1 gig microsd card. R4 alone should cost 40+. You can get it at SLS. (Please go with an experienced shopper)
You can play all NDS games as long as you can download it. Can play music and movies too, can also modify themes (my bro got a spongebob theme, damn cute). Constant updates for R4 can be found on the web. Easy UI too.
Have fun with your DS, DS FTW!
EDIT
R4 is a small square size DS card to be inserted at SLOT-1 of your DS (Upper slot), it's fully flushed so don't worry about it being ugly or pops out of your DS slot.
All you have to do is to install the R4 into the Card and then just download the games and place it in.