You should wait for ivy bridge. By the time you get your sandy and get your computer all set up and working, that new chip is going to come out and you're going to start hating yourself, no matter how little the improvement over your current processor might be.
All I can say is that the new i5 Ivy Bridge is going to run cooler and more then likely will have better Overclocking. If you have OCing in mind I would wait. If not then you may be able to find a 2500 none "K" for even cheaper.
By the way don't worry about panic buying, the 2500k is a fine CPU and you can overclock it over 4Ghz. Who really needs a CPU that's faster than 4Ghz anyway, really? It's fine, you got a good deal so fair enough.
It isn't about OCing headroom. It's true you won't see a gain above 4.5GHz, but since IB is priced similarly to SB, it is an offer you'd be foolish to refuse. Less power usage, heat consumption, PCIe 3.0 native support, and USB 3.0 native support for no price difference is an excellent deal.