Core i5 or core i7??

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.

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.

Both chips are great performers in their respective price ranges. The 2600K is about 20% to 25% faster in non-gaming applications due to hyperthreading. They perform almost identically in games, as hyperthreading is useless for gaming.

Since your a non-gamer, the question is simple: Is the extra $100 worth it for a chip that is 25% faster? Whatever answer you give to that question reflects the chip you should purchase.

Assuming the programs have been designed to take advantage of HT.

This isn't 10 years ago. The vast majority of non-gaming apps receive some benefit form hyperthreading. And 20% to 25% overall benefit is what the benchmarks show. I didn't make the number up.
I'm not questioning the 20% - 25% performance gain. I am just stating that as long as the program has been designed to use HT then there will be a performance gain.

I don't really use that many different programs so if a lot of current programs make use of HT, then so be it. The only programs I use that I know can make use of HT are MS Office, WinRAR and Handbrake.
Hyperthreading is not useful unless you do video rendering or transcoding. The i7 offers very little real world performance difference compared to the i5.

Photoshop will not use it. You'd be better off buying a bulldozer cpu than the i7.

The i5 is a pretty amazing cpu, it should be more than sufficient for what you need it for.

If you want a fast system, get plenty of fast ram, 8gb or more of ddr3 1600 as well as an solid state drive. You should be able to get these from the saving of not getting the i7.