Hello, my name is Dan.
I will do my best to help you, the only stupid question is the unasked one.
Which version of OS X did you have before doing the update to 10.6.8?
Was there an error saying that CS was not able to run? Or was it just 'Unexpectedly quit'?
Did you prepare for the update? Apple doesn't say much about how to do these things and they should.
500GB is a whole lot of RAM, more likely it is 4GB RAM and 500GB hard drive, yes?
Just kidding around.
Parallels is a vital machine made to run other operating systems.
I use it myself, there is very little interaction except for file sharing between Mac and Parallels.
CS5 is a universal application made to run on Intel whereas CS2 is made for power PC, that is what Macs were before going to the Intel chip, there is a way to revive CS2 with Rosetta,
That should allow CS2 to run, better yet (I know this will cost) is an upgrade to CS5.
I don't use Adobe as a rule but I like PhotoShop Elements 4, the rest just doesn't do it for me.
I like Pixelmator (look in the App store for it), it is as good or better than PhotoShop IMO.
One thing that will help performance is repairing permissions, this PDF explains.
Does that help?