This may have been fixed at some point, but I recall that under the original Rosetta under Tiger, there were strange precision errors in CS2.
With the RAM, make sure you get it in 4 sticks. Since the Mac Pro is quad-channel, you definitely want at least 4 sticks. Ideally, you want exactly 4 sticks (to avoid the higher latency on the back-4 sticks), and ideally they should be the same size (but this is less important).
Another thing to consider is hard drives. While I'm not suggesting moving to SAS, getting a Raptor or even moving to a RAID0 (with Time Machine on a 3rd, larger volume) will also help performance. I have a dual-dual Mac Pro, and the RAID0 I have noticeably helps speed.
Can you please tell me in layman's terms what this does and how it speeds the machine? My wife's running a G5 that's 4 years old and a little slow, and only has 4 gig.