I'm not sure I'd trust them but wccftech
claimed they benched an engineering sample ivy bridge i3 a few months ago. It might give you an idea of how they compare at least.
It's only slightly faster at the same clock speeds but would use less power.
Bear in mind that if that is an ivy bridge chip it's a much earlier revision than the retail chips will be and the retail chips might outperform it.
The IGP is where Intel were aiming to make big gains this time around so it should be much better on the newer i3s.
They moved to finFETs (more efficient transistors) and to the 22nm manufacturing process from 32nm (also more efficient) so Ivy bridge should be much more efficient than current CPUs.
Really, whether or not to wait is up to you but I wouldn't wait three to four months for it if I were in your position. Well, not unless I really needed more GPU power. If you are going to use that PC for gaming it's a tough decision so I'd probably buy the sandy bridge i3 now and swap in an ivy bridge part later if you can afford to.
The ivy bridge chip should be similar in terms of CPU performance. Probably a little bit worse but it's unlikely to be more than 10-15% unless Intel increase the clock speeds by a lot.
It will have far better graphics processing power though.