To answer your question about when does the beach ball appear, I have to say it appears all the time.
Do you leave your mac on 24/7? There are a number of cron jobs that run at nightly, weekly, and monthly to keep your file system in order. Shutting the computer down will circumvent this maintenance. Also, you might try running Disk Utility (in Applications:Utilities folder) and do a "Repair Disk Permissions". If the problem persists, then you might have to boot from your installer disk and run Disk Utility from it to do a "Repair Disk". The only times that I get the beach ball is with some web sites that implement a non-standard version of JAVA and when Rosetta's been idle for a while.
Photoshop CS runs faster on my Intel Mac Mini under Rosetta than it does on my G5 iMac. Not sure why yours is running so slow.

