How much graphics memory on the 285 and the 650?

DX performance will be better on the 650 but maya does not really take advantage of DX and Maya does not take advantage of cuda cores.

Moving from the 285 to the 650 is a bit of a give and take as far as Maya performance. The graphics architecture of the 285 is based on Quadro and I have seen a significant drop in performance as I am modeling especially with 2-sided lighting on, with both the 480 1.5GB, and the 570HD 2.5GB card I have now.

However, if you are getting more graphics memory, better DX performance, and the ability to take better advantage of cuda based applications.

I'd say give it a shot and if you run into any performance problems you can always swap graphics cards with your sister.