Really depends on file size, number of layers, etc.
Using a 21mp raw file processed to a tiff with 7 layers as a baseline. No. Gains will be real, but minimal adding a second SSD for scratch, vs. designating part of your main SSD for scratch and page files. I'd say even with 12gigs the answer remains the same. You won't be using more than 3.2g in any event unless you're making the move to CS5 x64.
You'd make more significant gains by raiding two SSD's (exact like SSD's) and using them the same way you'd use a single SSD. Disadvantage no TRIM support.
I think as a better option.. if I was building today.. I'd look at
OCZ's Revo drives. MUCH faster.
Either route you take.. you'll make more performance gains by ensuring proper setup and buying the right SSD.. than adding a scratch disk. I'd also recommend you plan on leaving 30% empty.. after 30% is passed you start to lose performance in the same way 50% exceeded on a traditional HDD would lose performance.
And todays 2-3tb SATA II drives.. are significantly faster than last years 1-2tb drives.. so when buying get the latest models. 30-40% differences in file access time assuming you'll be using these for data storage.
