Tough to say. In general, I wouldn't say upgrade the CPU. A Phenom II just isn't going to out-perform an Athlon II by much, particularly in gaming. On the other hand, Diablo III is going to need more CPU power than GPU power (it's a blizzard game, which means you can pretty much run it on a napkin with a GPU drawn on it). If you upgrade CPU, I'd look at an Ivy Bridge i5 or an i3. Otherwise you're just not getting more oomph, as the Athlon II is a serviceable if not great CPU.
The problem with the 6850 is that it's decent, so upgrading it significantly would mean moving up to at least a 7850, which costs a fair amount... more than an i3 + h61 mobo. It looks like you have a good monitor to take advantage of a better GPU though... tough call.
You can see the minor benefit of the Phenom over the Athlon in gaming here:
Old review, but applicable. Those benchmarks are for x2s, but the same holds true for x4 versions of the two processor lines.
But if you look at intel processor reviews, you'll see they have a much bigger edge over the Phenom IIs. If you're happy with current performance, I might wait until the new games come out. If more Diablo III performance is what you want, get an i3 or i5. If more shiny textures in a non-Blizzard game is what you want, get a better GPU.Edited by MisterFred - 4/24/12 at 11:02pm