As an Apple follower / nerd by trade, let me try to help you. There is going to be an Apple media event today, so any updates that take place to their notebooks will happen today. They keep on schedule - the laptops update around this time every year.
There is no longer any 15" white MacBook. The only white MacBook is a 13", and is every bit as capable as their higher spec'ed and higher priced counterparts, so as a new buyer, don't let them upsell you by some magic fancy smart talk.
The 15" laptop is an aluminum MacBook Pro. If the only thing she cares about is screen size, this is your only option. The 13" is, as I said, basically the same innards, and does all the same stuff, just in curvy white plastic.
Incorrectly stated above, the white plastic does not get dirty or yellow easily. The white MacBooks are a polycarbonate plastic, which does not yellow nearly as easy at all. Cleaning the OUTSIDE ONLY takes a simple light scrubbing with Mr Clean Magic Eraser. The plastic will also not dent like the aluminum if it takes a light fall, perfect for the young crowd.
Macs are notoriously not great for gaming. Today, they are perfectly capable, but the problem is finding the games. The upside to this? Something PCs can't do - Macs can run Windows, so you can have 2 computers in 1. That way, she can game on Windows, and do everything else on the Mac!
Anything else? I subscribed to see replies. I thoroughly enjoy helping people with technology. :)