I'm going to parrot pretty much everything said so far. Going from a Q8400 to either a 2600k or 3770k will be very noticeable. I was in a similar quandry as yourself on whether to wait and get a 3770k or 2600k now. I shopped around and got a good deal on a NIB 2600k, so I went with the 2600k. I pocketed the money saved (roughly $100). Now I will wait for Haswell.I have a Core 2 Quad Q8400 with G41 chipset. I was planning to buy Intel's new Ivy bridge, i7 3770. Since it is said it will be priced similar to the present SB is it worth the wait? What is the future ahead after Ivy Bridge?
Will I see significant performance differences between Q8400 and i7 3770?
Basically it boils down to this:
If you are going to pay full price for the CPU's, then get IB (5-10% more performance for close to the same money). But if you can shop around and be patient, you can get a deal on SB.
If you don't OC, get IB (5-10% more performance). If you OC, you might want to wait see what the real reviews say on IB, then make a decision.