Stacking is a snack in CS3, photo merge is much improved, I think HDR processing is also improved. There are some other additional tools in CS3 that aren't in CS2, which make life easier and quicker (you can proably still do it in CS2 but with more work on your part).
Although I use CS2, CS3 I understand has some really nifty selection tools and a "squash" filter that does a much better job than the min filter in CS2 for making stars smaller.