Actually, you might be light on RAM. You said you have 768 megs.
OSX, like XP, eats up a lot RAM & VRAM. We don't run a graphics machine with less than 1.25 gigs of RAM. When you run out of physical ran, OSX is gonna go to the hard drive and draw virtual memory. Things slow down a lot at the point.
If you want to see how much RAM you're using go to Hard Drive/Applications/Utilities/Activity Monitor.
Right now I have Safari, Entourage, Acrobat pro & Indesign running, but no docs open other than this safari window. Activity Monitor tells me that I'm using 729 megs of RAM. Indesign is usiing 118 megs with nothing open. Acrobat is using 80 megs with nothing open. Safari is using 52 with 1 window & 1 tab open & Entourage 22 with nothing open.
Open a simple 4 page ID doc, throw in some eps and tiffs, drop shadow some test and watch your Ram usage jump. Open Illustrator at the same time, do some fancy stuff and watch your ram usage go up more. Then open a 600 dpi Photoshop file and watch things soar.
In the meantime, you can go to the ID View menu/Display Performance and check your setting. If High Quality, you can reduce that & turn off Object Level settings. That will speed things up, but things won't look as nice as you work.