I just upgraded from CS3 to CS5, mainly for the multiple artboard feature. CS4 users most likely could skip this version and not miss much, but there are those technophiles out there who always like the latest and greatest.
The multiple artboards are absolutely awesome. My typical customer template consists of about ten artboards at various sizes. Regular 8-1/2 x 11 for artwork approvals (scaled down artwork) and larger sizes for working on full scale designs or mockups.
I have several artboards set to the exact size of my plotter media size which I usually export as .dxf files directly into SignLab 8. It has a feature that allows specifically selected artwork to be exported instead of the entire file...very handy.
The best part about it is no more having to minimize and maximize Illustrator windows when working with multiple files...they're all in the same file, and each is cutable and printable only when needed...no more moving your design on and off from the artboard.
If you do need to use multiple files, the tabs are always docked across the top of the window...another one of my favorite features.
If you're a new learner, and the savings for CS3 is significant, then by all means start out with CS3. You can up grade directly to CS5 whenever you get the urge, but it must be before CS6 comes out.
If you expect to be a "power user", then you might want to go for CS4 or CS5.
JB