I'd say has you already have CS4 that Lightroom is overkill and expenesive just for the image managment part of it.
To be honest I have LR but never use it for image managment as I much prefere something like ACDsee which is lightning fast,
a lot cheaper and has a better cataloging and search facility in my opinion.
But what ever you choose to use from piccasa to LR forsetti is correct about keywording being the means to good image managment.
For example I have over a thousand images of just of my three Grandchildren, Brandon Chelsea and Madison and some with their father Grant.
All my images are keyworded so if I wish to find images that contain only Chelsea and Madison but not any that contain Brandon or their Father, then all I do in ACDSee is type in " chelsea+madison -brandon -grant " and in exactly 0.5sec (I timed it) it displays just those out of thousands of images.
Of course it pays to start before it all gets to unmanagable without a lot of hard work.