I'll think there'll be a speed boost, but for only the hard drive it's probably not worth the cost. It's the other uses of SCSI that make it worth holding on to: Cheap scanners and easy connection of a CD drive or additional hard drive if you ever need one.
A CD drive on a full SCSI bus will be much faster than IDE or PCMCIA->SCSI.
Easy access to an additional hard drive will be most welcome if you ever need to do backups, salvage files, emergency boot, etc.
