Good choice for Pokemon Diamond. i got Pearl a few days ago it it is GREAT!
Some great DS games:
New Super Mario Bros. (no Wi-Fi)
Animal Crossing: Wild World (Wi-FI)
Star Fox Command (Wi-FI)
Mario Kart DS (Wi-Fi)
Pokemon Diamond/Pearl (Wi-Fi)
these are some of the best DS games I own (in no order) Also I heard Final Fantasy III was good, although I think Final Fantasy is overrated.
Hope you choose some good DS games