I think a combination of the suggestions would be your best bet. DNG files are RAW files, but are that little bit smaller than .cr2 or .nef etc, over a lot of images that can free up a fair bit of space.
Are you keeping PSD or TIFF files? They take up a lot more room than RAW files. Likewise do you keep a lot of jpegs, I find that it is just as easy to regenerate jpegs as and when I need them and when they have been used to delete them.
Chappers suggestion of going through your images and deleting duplicates etc is a good one, I did this and got rid of 1200 images out of my Lightroom catalog (1200 * 6Mb freed up a lot of space) and I need to do that again. After 6 months I only keep anything that I have either used, or think I am likely to use again. (Although all RAW files imported into Lightroom are stored externally). This way I'm able to work with a 120Gb main drive without too many problems.
You can partition your time machine backup, it is good to have your back up drive a bit bigger than your main drive, that wa you can keep a bit of history, but you could easily take 500Gb for other storage. I copy my RAW files to my time machine drive as I import them, so even files deleted from Lightroom are still there if I ever need them, although that hasn't happened yet...