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There's a good suggestion that Scott Kelby puts at the front of his LR3 book that I've found worthwhile:

"Keep all of your photos inside one main folder. It doesn't matter how many other folders you have inside that main folder, just make sure all your photos reside within it. This is the key ingredient to making everything work smoothly. If instead, you import photos from folders in different locations all over your computer, you're headed for trouble."

He suggests creating folder inside your Pictures folder, and naming it "My Lightroom Photos" and import all your pictures into there (or move your RAW files into there before setting up your LR Catalog).

That way, if you ever need to move to a bigger disk, you just have to copy or move one folder.