Adobe has posted a very detailed and thorough document explaining how to fine tune your PC for Photoshop CS3. Bottom line is (as always): get a fast processor, a ton of RAM, and a RAID 0 array for your scratch disk. Even if you have that, there are quite a few things you can do to optimize your workstation... Cache level preferences, memory allocation percentage, etc., etc. Well worth a read if you spend any significant amount of time in PS.
Adobe - TechNote : Optimize performance of Photoshop CS3 on Windows XP and Vista