Is the core i3 enough?

yes especially if you can over clock it a bit. Both WoW and SC2 run on two cores and do better with Intel then AMD (see the link below). Since Diablo III is also a blizzard game, it should be a similar game engine in terms of hardware demands.

http://www.techspot.com/review/305-starcraft2-performan...

i would not suggest getting an i3 right now, sandy bridge is less than a quarter away, the cpu is quite capable, beating even q8 series in some tests as well as the athlon II x4, but the onboard graphics are not appropriate for new games, i get good framerates on NFS most wanted, but i would not recommend anything newer, the onboard gpu only has 12 shaders

it is doubted that intel will release hexacores for 1156, since that would sabotage 1155 sandy bridge, wait for 1155 or go amd tripple core

I wasn't planning on using the onboard graphics, perhaps I should have stated that in my original post. Would the sandybridge CPUs use the same motherboards as the core-i family, or would I have to upgrade it as well as the CPU when sandy comes out?

the sandy bridge due to be released will be socket 1155, and all reports so far say that neither cpu nor board will be backward compatible with 1156

gotta love intel

Gotta screw with money, intel..