If you had CS3, stay with it.
If you had neither, it would probably make sense to start with CS4, unless CS3 was already bought, or cheaper, or in some way more desirable.
CS4 rearranges the GUI some, some supposed new speed tweaks (though I have haerd mixed reports on the GPU Acceleration), not sure if any new filters were added.
I remember using 7, then using CS2 on someone else's PC, and being totally lost. I don;t think CS4 is such a change from CS3 (as from 7 to CS2), but I still think if you had the opportunity to learn, laern on the newest. I know one thing I tried with CS2, one of the transform filters, was much improved in CS2 (you could move several points, not just the 4 corners).
If you are just doing some photo work, CS/2/3 should do just fine, even PS-7.
Main thing I'd look forward to is the content aware scaling, some of the 3D stuff (which only comes with extended, which is like $300+ more), the GPU acceleration and some of the image zoom features (smoother zooming, apparently, ability to rotate canvas to work on it without having to rotate the actual image, a grid shows up when zoomed in REALLY CLOSE, some other things).