How much do you have now, 4 x 2GB?
do you need 24GB to handle work in CS5?
2009 model year? MacPro4,1?
Crucial and Kingston are excellent choices especially for UK.
Kits of three triple channel - use to be that you couldn't mix 4GB and smaller, same holds true for 8's, and you might want a pair of 2 x 8GB
OWC has a rebate / buy back of Apple memory
RAM timing is determined by the processor's memory controller and Apple firmware, and you may be able to use 1333 but it may not run @ that speed (check OWC page)
Then I assume you have a single processor and 4 DIMM slots, 3 x 8GB would be ideal but won't mix with what you have.
I wouldn't bother with 4GB DIMMs, even if you go with 2 x 8GB for now.
SSD for boot drive is a nice. And look at scratch, disable Spotlight for scratch, all the usual tips and tricks optimizing photoshop. A fast boot drive is one upgrade most feel returns a great overall improvement in performance.
I am in the same page as Dareell, I am planning on upgrading my memory from 6GB. I have a Mid 2010 Mac Pro 5,1, and I am curious if its good to max it at 16GB of RAM. I am a recording engineer (and PC gamer [bootcamp with Win 7 x64 and Radeon HD 5870]) and I am curious if maxing it out WILL NOT make a performance issue.
What is your suggestion?