I know there are songs that DD couldn't play on her 61-key. An 88 key would mean you'd never run into that problem. So, the 76 is a step up, though she may want to play something that she's not able to, because the note simply isn't there. Also, weighted keys were a "must have" for my DD.
DD has this:
http://www.musiciansfriend.com/keyboards-midi/casio-cdp-120-digital-piano
We got it on / around Black Friday last year with a nice stand and bench for $400. She is in a band that plays at local restaurants / fests so we transport it around a lot and it has held up well. It's even been (lightly) rained on a few times during some outdoor gigs this summer. Also, other members of her band play piano (she is mainly the singer...only does guitar or piano on a few songs) and practices are always at our house, so she is not the only one using it. Basically, it gets a lot of use!
Oh, wow! I just hopped onto Costco.com to see if I could see the 76 key you were possibly looking at and found this....
http://www.costco.com/Casio-CDP220-88-Key-Keyboard-Bundle.product.100058113.html
Seems like a great deal and very similar to what we have. If you were looking at the $299 76 key, this is only $50 more for an 88.
She is more into her guitar, too at this point, but seems to go back and forth.
Good luck!