Best of both worlds with teh cards:
Get an R4 for slot one, which will do just about any homebrew worth using (only the crappy stuff hasn't been updated for DLDI). And it does DS roms better than anything else around, pretty much. Price ~ $45.
And for GBA roms and DS web browser, get an EZFlash 3-in-1 expansion. It's DS Lite sized, so it obviously won't work with DS phat. There is a homebrew that you must get which makes it both read in English and work with R4. This app is by some guy named Rudolph or something similar. There's info around; look it up more yourself. Price ~ $20.

Slot two cards are expensive (Around $70 - 80) and they don't work with all roms. If all you want is DS, stick with R4. But if you want GBA as well, you might as well get the expansion thing. It works real nice, and depending on what you have to pay for shipping, it can be cheaper than getting M3 Perfect Lite or similar product.

And while everyone is talking about Ouendan and EBA, I like Ouendan better just because I happen to hate most punk, hip hop, and motown, which includes pretty much every song. And Cher. Putting her crap on there really did not help. Ouendan, on the other hand, had stuff that I did not recognize and was too busy playing to learn to hate. But if I actually listened to J-pop, my opinion may have been different. All I know is that once Ouendan 2 came out, EBA went right off teh R4 card. Haven't played it since.