I started with CS5, after learning how to apply effects and layers in Paint.NET CS wasn't very hard at all. I downloaded variety of layer styles, brushes and applied text and those brushes on stocks I already had. I don't think using CS4 or CS5 is a good idea for beginning though. Most of the tutorials on the internet are too overall and may not suit you but try to browse through filters and left-click tools which enables other tools with the same kind.
So, at an overall for a beginner I think;
CS2 - 7/10 - Has very less features and you may not be able to do variety of things
CS3 - 9/10 - Can be a very good choice as still most of brushes and layer styles are CS3 compatible, also doesn't have enough features but enough for a learner
CS4 - I never tried it so I won't vote for this one
CS5 - 8/10 - Although I would vote this 10/10, it can be very complicated for a zero-beginner.
All of them has tons of pros & cons but as an overall for a zero-beginner CS3 would be good. If you want to go for CS5 or CS4 at first, you can give Paint.NET a try. After downloading 20 plug-ins (20 plug-ins are about 5mb, don't worry

) it can be very usefull. I used to do every single signature/avatar on Paint.NET for 9 months - I still use it for resizing and small effects as I'm still used to it's very simple UI