Its not cheap, but two processors are the way to go if you want big photoshop performance, people think dual core is as good but it is not as the cores share the same bus

but if your looking for big improvements which are cost effective simply run your OS in ram on a device like the iram drive.. have a look at the vids for how much quicker it is.

XP on iRam
http://www.youtube.com/watch?v=zShWv...eature=related

Vista on Iram
http://www.youtube.com/watch?v=kofpP...eature=related