depends how you work with your images. If you use alot of filters it offers smart filters which can be re-adjusted later in the edit (just like in a video editor) and the new photomerge and merge image features are great.
Most of the other upgrades or improvements are a bit lame, the new contrast and brightness is a big improvement on the old ones but most of us don't use them anyway, levels and curves still offer the best options here.
curves is improved with added histogram like in levels.
The new quick select brush is a cross between the magic wand and the brush tool and is very heavy going. The pen tool is still the best option for selections. The ability to refine your selections using mask view.
The raw converter and bridge are improved, the raw converter looks to have the develop module as Lightroom.
Improved vanishing point.
Unless there is a specific tool that would make life easier, then I don't think CS3 will improve your pictures unless you make panaramas, which is actually a huge improvement in CS3.
Resizeable palettes and some other aesthetic stuff.
There's more but I don't remember it all off the top of my head.
Hope this has helped and not made things worse :shrug:
Mike.