It depends on how the program is coded, im not sure exactly sure if CS5 is highly threaded.
http://www.anandtech.com/bench/Product/288?vs=287
That is the difference between the two, as you see they are quite similar.
Yeah CS5 apparently is a game changer when it comes to hyperthreading and 64 bit and all that jazz, so CS4 benchmarks may be a lot closer between the two CPUs than CS5 benchmarks, which I can't seem to find.
Kingkazma, you've been saying 2600k is worth it over the 2500k from the get-go, have you got any word on what specifically we're talking about here in measurable performance?