What you are modifying is the boot.ini file. If you are concerned, just copy of the line of the boot ini where you are going to add the /3Gb. Now on boot you will get two options, to either boot up with the 3GB switch or without it. Net, if you get into trouble, you can always get back to your box by selection of the non 3Gb boot.ini.
I use the 3Gb on all my machines that can run 4Gb. I agree with Jack that the actual increase in performance is nominal. I have always felt that CS2 (haven't loaded CS3) seems to start out fast but over time bog down like a memory leak is happening. So having the /3Gb switch on delays this for a period of time.
Also it's my understanding that in a windows world, CS2 can only address around 2.5GB of ram, no matter if you are running a 64 bit version of XP/Vista or not. I have one machine running XP 64 bit edition, and windows will see all 4Gb of ram, but CS2, can only take around the same amount as my 32bit Xp machine. I don't know if CS3 fixed this or not but it would be nice to know for that would make a good fit for my 64 bit machine more than my current 32bit XP machine.