Core i3 Sandy up to Core i5 Ivy probably is worth it (performance-wise), in the future if not right now. Personally, though, I'd probably wait and keep half an eye out for a really good deal on a Core i7 Ivy. Or I wouldn't upgrade at all.
I don't see much point in half measures. If you're worried about core counts going forward, then the ~30% practical core-count boost from Hyperthreading is not to be ignored. On the other hand, if you're satisfied with the i3 as of right now and for the foreseeable future, you're probably best off riding the i3 as long as you can, and then going whole hog on a new mobo/CPU from whatever generation is current at the time.
On the other other hand (hehe), if Intel's current rate of progress holds true, you won't see a truly massive boost in (per-core) performance for probably 3-4 generations -- and there is a practical limit on how much various applications can be multi-threaded. So a current i7 may perform competitively for a really long time.
As Someone Somewhere says, the matter really rests on the OP's usage habits.