Gaming will be her only real issue and it depends on how serious of a gamer she is going to be. The graphics card will be more important than the processor for gaming since a 2.4 GHz will run most games. Also the 2.4 is not the same in the Core 2 Duo as it is for the new Intel iChips. Here is a review at MacWorld about the 15" i5: Apple 15-inch MacBook Pro/2.4GHz (Core i5, Spring 2010) MacBook Review. It shows that the i5 is even faster than the 2.66 GHz Core 2 Duos but as I've mentioned the processor speed is not necessarily the most important thing when it comes to games.

The 15" has a better dedicated graphics card compared to the 13" but I haven't read the reports on how much better so you might want to research that yourself. On my MBP I can tell the difference between using the slower integrated 9400 card compared to the dedicated 9600 card. The 15" have dual graphics cards if you didn't know. If your girlfriend wants to do gaming in the future then the 15" would be the better choice. It's not that the 13" can't do gaming. The 15" will just do it better so it also depends on how high of a quality your girlfriend wants to play the games and if the compact size of the 13" is important to her at all.