You can have 2 versions of PS on your computer - just don't uninstall PS6. That's not a problem at ALL - don't worry.

As far as the brushes working with CS3...? Kind of a crapshoot - maybe, maybe not... Gonna have to see to, well... See...

And like you were hoping, if they don't you'll still have PS6 on your computer for those certain brushes and filters and actions and whatever that you're used to.

Now here's the question - CS3 Standard or Extended...?

hope this helps-
-Lew ;-)