Expanding the RAM is good for a graphics guru :). Matched pairs is also a very good idea. I've used crucial.com in the past without incident, though some others claimed problems there and one of my friends went with datamemorysystems.com instead.
How are you doing on hard drive space? If things start getting tight, then the computer might slow down when swapping virtual memory to disk. Try to keep at least 2x your real RAM available.
Check what kind of start-up programs you have running--start Activity Monitor, sort descending by CPU, see if something is using up lots of CPU. It's unlikely but there might be a background daemon installed by some other product (e.g., Google Earth installs Google Updater and doesn't tell you; Updater takes up a small amount of resources, but if you had a number of such things they could add up).
Are you up to date on system software? 10.6 claims to be a significant speed improvement.
Not huge things, but stuff to check
How are you doing on hard drive space? If things start getting tight, then the computer might slow down when swapping virtual memory to disk. Try to keep at least 2x your real RAM available.
Check what kind of start-up programs you have running--start Activity Monitor, sort descending by CPU, see if something is using up lots of CPU. It's unlikely but there might be a background daemon installed by some other product (e.g., Google Earth installs Google Updater and doesn't tell you; Updater takes up a small amount of resources, but if you had a number of such things they could add up).
Are you up to date on system software? 10.6 claims to be a significant speed improvement.