The value of an upgrade depends on what new features are in the new version and is it of any value to you. I had CS and didn't upgrade to CS2 but did for CS3 as there were a couple of new enhancements that made it worthwhile.
I think CS4 is available as a 30 day download now so you can try it. There are a few sites that have some of the new features as tutorials, have a look and see if it's worthwhile.
There are a couple of things I think may be useful but I'm not rushing at the moment, as I want to see if they make the upgrade worth the cost