With most computers, a larger amount of RAM will provide a larger benefit than a slightly faster CPU. The two types of programs that benefit from having a fast CPU are games and video processing.
I don't know enough about the current image editing programs and know nothing about the processing of RAW images. I would think that a faster CPU would help, but getting one model down (3.8GHz vs. 4.3GHz) shouldn't be a huge performance hit.
And the number and type of programs running in the background also makes a big difference, as they tend to steal CPU cycles. I generally try to run as few programs as possible so that programs get as much CPU power as possible.
In the past, antivirus programs were big CPU hogs. I don't know if that is still true today. I'm not as big a PC geek as I once was -- although I did build myself a new PC last year, and I was able to get WinXP running in a virtual window under Windows 8 using VMWare.
By the way, I have a Rollei Magic II tucked away in a box somewhere. Not an impressive camera at all. Somewhat ugly, too.
Also, Roger, I think you are one of the best photography writers around, having read your columns in AP for years and also having bought several of your books. I hold you in very high esteem.
Even for processing big image files?
I'm not arguing. I don't know enough to do so. But I'd always assumed you'd see a modest improvement.
But to the OP: from limited experience, and it's always a personal question, the f/3.5 seems to me to have a 'magic' that the f/2.8 lacks.
Cheers,
R.