I don't believe PhotoShop makes use of Hyper Threading so if that is the program you use most often, then there is really no need to buy a Core i7 CPU.
You can read the following article about PhotoShop performance, however, it is based on the Mac version. But the performance should still apply to the Windows version.http://macperformanceguide.com/Reviews-MacProWestmere-Photoshop-CoresSlower.html
You should do research into any performance issues on your own with regards to Hyper Threading and PhotoShop. Perhaps join their forum to find out if anyone can confirm or deny any potential gains in performance.
In the following list of benchmarks there is only one benchmark for Adobe PhotoShop CS4 called Retouch Artist Speed Test. The Core i5-2500k (3.3GHz) completed the test in 12.6 seconds. The Core i7-2600 (3.4GHz) completed it in 11.3 seconds.http://www.anandtech.com/bench/Product/288?vs=287