All depends on the games she'd be interested in playing on it.
If she doesn't already have one, my guess would be she's not exactly a hardcore Halo/Gears of War fan.
It's not a bad idea to have one for RPGs, with Fable 2 and a bunch of JRPGs currently available. If not... well, they're only $200. She could always buy a 60GB net pack for $100 later if she's interested in using it for a lot of online downloads and such.
Then there's the whole issue of whether she wants to be tied to a subscription service with XBL. If not, then it probably won't be seeing a terrible amount of use except for when she picks up a new game.
It's still easier and more convenient to use than a PC for gaming though. I don't know how tech savvy your sister is if that's an issue.
A lot of the DRM issues with PC games have made me weary about buying any games through retail rather than on Steam. These days, if I know a game has DRM/net activation/limited installs, I won't bother buying a retail copy. Too much hassle.