no experience with cs3/cs4 or lightroom I'm afraid so can't offer any advice stemming from on-hands experience but.... apart from whatever new tools though I heard cs4 is essentially a 'faster photoshop' - it has inbuilt graphic acceleration which apparently means you spend a lot less time waiting for screens to load/commands to complete.... also instead of only being able to properly judge an image at (for example) 12.5%, 25%, 50%, 100% due to anti-aliasing applied at other values, cs4 now shows a crisp image in spite of the zoom ratio.

Rich mate, even THAT wouldn't strictly be a reason for upgrade - my 40d raw files aren't recognised by Bridge/Photoshop cs2 but I get around it by batch-converting them all to .dng files as soon as they're on the machine, then wipe the original RAWs. admittedly my average .dng file (lossless, with RAW embedded, just in case) is just shy of 20mb, but I guess it makes me more selective which ones I keep and I end up with a more satisfying selection at the end having gotten rid of everything just taking up space.. 

Adam Edwards Photography