Is there that much difference between the 2500k and 2600k?

What sort of applications would you be using?

What are you doing with this CPU? If you're playing games you would be better off with the 2500k. With the I7 you're paying an extra 100 dollars for hyper threading and extra cache, two thing that games really don't make use of right now. If you're doing something more CPU intensive like video editing or 3D graphic design you would be better off with the I7.

For games, nothing. For video transcoding, compression, etc there's a sizable difference depending on the app.

Go here http://www.anandtech.com/bench/Product/288?vs=287 and click View Comparison under CPU Product Benchmarks.