Log off does what a reboot usually handles. Though I'd rather reboot now and then and lose my uptime (25 days on my PB 17 right now) and gain speed back.
Cron jobs will do most of it. Caches are a killer. Kill those and the system can purr. And get more ram if needed.
Probably wasn't Grab. You can so shift-option-3 for a desktop picture. Shift-option-4 and you can select a portion of a window to take a screenshot of (by dragging), or shift-option-4 then the space bar where you get a camera icon to take a screenshot of a particular window open and only that...
Are you in Tiger? Go to Preferences > .Mac > Advanced and reset sync data so your computer overrides .Mac not the other way around (which is the default, especially when you have more than 1 computer on the sync list).
Empty your cache.
Empty your RSS feeds.
Edit Safari > Preferences > AutoFill > Others down to less than 10 and turn it off. Shut down Safari.
Never heard of Yasu.
Apple recommends that FileVault not be used on home computers. It is more for mobile usage. If you have files you want to protect, create en encrypted disk image using Disk Utility. It does the same as FileVault but without creating the sparse image which doesn't reclaim space.
Both Jeff and...
Mac not MAC. MAC means something totally else in computing.
Anyway, Preferences > Security allows you to lock the computer and you can even set times for it to sleep, shut down and wake up. In Preferences as well.
Zapping pram doesn't solve the Photoshop problem. It's a known bug. Only way to fix it is reinstall PS or add the missing pref file.
Also, 768mb of ram for PS? Not doing much are you?