I'd pick the E8400.
Photoshop uses 4 cores only in a couple of areas that are typically used in benchmarks, and you end up with misleading benchmarks. In real life, either Q6600 or E8400 will be limited by the hard disk and won't work at 100% and you'd get exactly the same speed with any of them.
If you run lots of other things in the background while gaming then get the Q6600.