Lightroom is an application that needs a lot of HD bandwidth in read mode since it constantly accesses the drives where its thumbnails are stored, and also very often the drive where the original raw files are stored.
The - mostly untold - truth is that LR only works well with a very fast storage unit, either internal drive, or external eSATA or SCSI320, preferably in Raid 1, 5 ot 6. Since internal drives are typically too small to keep for a long time the work of a pro or active amateur, this leaves us only with eSATA or SCSI320 external RAID enclosures. Since those typically cost at least 1000 US$, this cost should IMHO be factored in when deciding to invest in LR.
External USB, firewire or network units (NAS) should IMHO be avoided if your time is valuable to you. Using LR on my 1TB NAS was a nightmare, only second to using Bridge CS2 on the same set up.