asus p5b deluxe CPU upgrade advice

Its a 10 years old system sparingly used but want to upgrade CPU.
I mainly use this for photoshop & lightroom & while converting files CPU hits 100% where as ram usage hits max upto 6gb

As of now I have
E6300@1.86Ghz,
P5B-Dlx Wifi,
8GB kingston DDR2 800Mhz,
Corsair GX 450 PSU

I plan to upgrade to faster intel 775 based quad core Q9400 2.66 GHz, 1333 MHz FSB
(its available cheap since its a used CPU)
I have checked my BIOS & motherboard both will support this Q9400 @1333 MHz FSB.

My question is how much speed difference can I get if I upgrade to quad core which has high clock speed?
I did this CPU-z test & in multi core bench it shows quite a difference

you will definitely get better performance. These cpus are a work beast TBH. Can't say about photoshop but if you want to know performance difference look for cinebench score. You will know the difference.