1.) Going from 667mhz ram to 1333mhz ram, will I noticeable tell the difference?
2.) Can I insert W3680 into the dual processor trays of a 5,1?
I am a photographer using PS and Lightroom 4 most of the time.
From 667mhz to 1333mhz - Yes, there will be a significant difference in terms of the latency. You will really find the differences depending on what applications you run, for example, compilation of code, number crunching and or scientific apps. In answer to your 2nd question.. The answer is NO. The W series are designed to be run in Single-Quad processor boards only as W does not support dual QPI.
However, the reverse is not the same as you can run an X series processor in the single-quad even though they are more expensive as they are found in dual processor trays.
PS and I think Lightroom are not multiprocessor and or multi threaded capable apps, so therefore a Quad CPU would be all that you need.. but really since the w3680 has come down in price I would just go with a 6-core as its the sweet spot and it will be a lot more speedier in performance.
Plus, you will gain 6 cores and 6 virtual cores for a total of 12. Dual x56xx processors give you 24 cores physically - assuming its already 12 core based.. while dual 6-cores will be 12 total.
1.) Going from 667mhz ram to 1333mhz ram, will I noticeable tell the difference?
2.) Can I insert W3680 into the dual processor trays of a 5,1?
I am a photographer using PS and Lightroom 4 most of the time.
I also wouldn't waste my money on a dual processor system. Just get the fasted clocked system you can with as much memory as you can. You'd be amazed how fast Ps is when you give it 16+ GB of RAM.
I wouldn't it significant but you will notice a small bit of zippyness over your old system. The processor speed and storage media speed makes a much much bigger difference.
For example if you could rate zippiness is tangible terms of performance increase between the systems it might look like
Base MP1,1, 2.66GHz, 4GB RAM, 1TB HDD, 7300GT: 1.0 (base for all below)
Base MP1,1, 2.66GHz, 16GB RAM, 1TB HDD, 7300GT: 2.0
Base MP1,1, 2.66GHz, 4GB RAM, 128GB SSD, 7300GT: 2.5
Base MP1,1, 2.66GHz, 4GB RAM, 1TB HDD, 8800GT: 1.5
Base MP1,1, 2.66GHz, 4GB RAM, 1TB HDD, HD5770: 2.0
Base MP1,1, 3.00GHz, 4GB RAM, 1TB HDD, 7300GT: 1.3
Base MP1,1, 3.00GHz, 16GB RAM, 128GB SSD, HD5770: 6.0
Comp MP5,1, 3.33GHz 4GB RAM, 1TB HDD, 7300GT: 1.7 (* match base only mem & CPU speed differs)