I don't think your sister is a great gamer. The stock ROM is enough. Is not as speedy as Cm7 but it has less bugs. And the battery is better. I tried CM7 but I come back to my default ROM (in Romania 2.3.5 it the official one). Just root it, delete 7player, task manager, cardio trainer and zinio and install RAM manager from android marker. The free version has just balance profile but is enought. Is not a app killer because this drains bat but optimise the RAM usage (less lag). Believe me, it works. The developer is one of us, a XDA member.
For CM7, you should wait for the final version.
For a girl, Xperia Ray looks better than Defy. I asked my girlfriend and she told me that. For me, as a male, Defy looks better. Also, Ray will receive ICS update...official. But that's not very important for a girl: she will want a good looking phone, without functionality problems.
Nokia Lumia 520