Oh crap, how'd I forget that!? I love Advance Wars! You can also pick up the two GBA ones for like $6 at the most. Fire Emblem is great as well, although the really good ones are on the GBA. The DS one was good, but the GBA ones are better.
If you decide to get Fallout 3, you should get the Game of The Year Edition. It costs a little more but it includes all 5 expansion packs which would cost quite a bit more if you bought them separately.
360: These are very long lasting RPG's. Oblivion, Fallout 3. Cheap and you can spend a huge amount of time with them. DS: This is hard. Hmm... Need For Speed: Pro Street is the best (and quite long lasting) racing experience there is on the DS(apart from Mario Kart). GTA: Chinatown Wars. Pokemon! The World Ends With You is awesome but I don't like it anymore since I'm stuck at the first real boss .( I suck.) Metroid Prime: Hunters. Try Final Fantasy. E: BTW, bought Oblivion Game Of The Year for 8,50 Euro's on Steam (PC version) .