Well besides the main first party titles (OoT 3D, AC:NL, Mario Kart 7, Fire Emblem, Pokemons), there is a new Phoenix Wright game (download only) which was is fantastic, Bravely Default which will be out soon and getting rave reviews, and is sort of a successor to Final Fantasy, Theatrhythm Final Fantasy which I played the shit out of - and I hate rhythm games, Kingdom Hearts 3D, Metal Gear Solid 3 3D and the list goes on and on.
If you getting a 3DS, I'd have to recommend an XL if you have even slightly large hands. For whatever reason the 3ds normal gives me cramps really quick in games when I never got any with the ds lite, although I'm sure you could try out both before buying.
Only problem with 3DS is the battery life still sucks, although at least they have fixed the issue where the battery still drains pretty quick during sleep mode. My XL can go about a week or more in sleep mode without turning on the battery warning.
Otherwise the XL lasts like 5-7 hours during gameplay. Pretty bad, but not overkill I guess. I hear they don't sell the 3DS's with chargers in Europe (and I assume the UK)? Hope you got a spare one if you do purchase.
Didn't ya have a DS?! But uhhh there is a ton. The million Nintendo first party games, a lot of the square enix titles everyone likes (chrono trigger, the world ends with you, etc).
Really you might as well go for a DS flash card that works on the 3DS and go from there, those aren't usually more expensive than a game. I have this one: http://www.realhotstuff.com/Supercard-DSTWO_p_25.html Just dump a rom on there and done. Works fine.
DS games kinda look like shit on the 3DS though unless you put them in normal resolution mode so they aren't scaled to the size of the screen. Sort of like playing a PSX or n64 on an HD TV.