Re: Scratch Disk and Cache Settings
The scratchdisk setting depends on the amount of harddisk you have installed. If you have only one disk in one partition the only option left is start-up disk. If you have a second internal drive installed and made this one a two partition with say about 40 GB for one partition you can leave this disk blank (don't save data on it) and use this one as scratchdisk only. This does increase the speed of photoshop. I don't think it is wise to make an external drive as scratchdisk but I have no experience with such setting.
With your amount of RAM you should be able to set the amount of RAM usage in Photoshop at a 100 %, this will result in about 1800 Mb use of RAM in Photoshop. I really don't know the effect of cache settings for histogram, I did set mine on at 4 levels but are not aware of any difference in speed (dual 2 G5 OSX10.3.8)