If your Aperture library has become larger than what your hard drive can handle, I have two possible solutions. The first is to employ this mobile drive solution in combination with tapping the power of the Projects feature in Aperture.
The second possibility is to try Ben Long’s Aperture Library Spanner that allows you to span an Aperture library across multiple volumes.
Regardless of which method you try, always backup (or vault) your entire Aperture library on an independent drive first. You can’t be too careful with your pictures.