Is there any good casual games for 360?

[Dolla Dolla]

If my memory serves me correctly, you can install the smaller arcade games on the memory card, no? Also, you don't have to pay to download stuff. You sign up for a Silver (free) membership on LIVE, and you can download all free content (demos, trailers, some free DLC for games, etc). Nonetheless, this list only contains Disc based games:

Lego Series (Batman comes in September)
Viva Pinata (1 and 2 - 2 in September)
Earth Defense Force 2017
Gears of War
All Halo games
Marvel Ultimate Alliance
Eternal Sonata
The Simpsons Game
Scene It!
Dance Dance Revolution
Rock Band
Guitar Hero
Lips (this fall?)
Kameo
Perfect Dark Zero
Beautiful Katamari
Dynasty Warrior Series
Pirates Vs Ninjas Dodgeball

You could also find someone who has a 20gb and is looking to upgrade, maybe they'll give you a good price. Then you'll have a much wider selection on XBLA.