Purchasing an iMac: when is the entry-level model insufficient?
It depends on what you / your girlfriend wants to do with the iMac. If it is really just used for internet and occasionally watching a movie and listening to music, then an 'entry level' iMac offers more than enough performance. IMHO even an iPad would provide enough performance…

We actually use our 5+yr old iMac (Intel Core 2 Duo, 2.66GHz) as a media hub and it works just fine. It's connected to the TV and has a couple of external hard drives connected to it. And when it got a bit slow a couple of years ago we just added some memory, both RAM and HD, and that really helped a lot. Now it's basically never  

Also all other apps like Photoshop etc. still work fine on that machine, but since we now have some faster machines in the house, the iMac is basically just used as a media hub.