Yup daveeb, you are right 137GB is the limit of LBA addressing in the Ultra ATA standard.
If you want to go higher than 137GB, one way around it is to buy an Ultra ATA/133 PCI card (e.g. Promise TX2). Your theoretical limit will then be 1PB (1PB=1,000TeraBytes).
Unless you are running Windows 9x of course, in which case you will be limited to 2TB. XP can address up to 16 Exabytes (1EB=1,000PB)
I expect Simon will be along later to tell you how much a Promise TX2 would cost at his favourite on-line store