Lightroom 2 is an excellent tool for tweaking your photo's and cataloguing them for easy access. It's got some very useful options like the digital Grad filter. There is nothing you can do in LR2 that you cant do in CS3 where image retouching is concerned, it's just in a much simpler layout and easier to use.
CS3 is more about image manipulation (cloning, adding colour filters, blending, etc) so using the 2 progs in combination (as I do) is the best way.
Download the 30 day trial of LR2 and have a play.