The Athlon II X4 620 is a dream for running Divx and Photoshop.
Since the Athlon II X4 is just a few bucks above the Athlon II X2, it's definitely worth it. If your sister only uses Photoshop, Office and the web broswer, except for Photoshop, the CPU won't make much of a difference.
VLC media player is very light on your CPU. Both CPUs will play acceptably. DivX and Visual Studio are both well suited to multi-core applications. In DivX, the quad-core will really shine. In Visual Studio, the dual-core will be faster, because of it's higher clockspeed and the fact it doesn't make use of all four cores yet. However Microsoft is amending this.
If you decide to run more than one of these applications at the same time you'll see more benefit from the Athlon II X4.