A conversation I had recently with a client (production house for a major advertising agency) and they are finding that scratch disks are making a bigger improvement on performance/speed than extra RAM. And these boys are using Mac Pro's with 8-Core, 12Gb RAM, 2x 2TB drives plus a 2TB drive as a scratch disk. One of the machines they doubled the RAM and saw no difference - adding a bigger scratch disk made a difference...
Could a USB flash drive me allocated as a scratch disk for out and about processing? - he types while reaching for his flash drive