When did you buy your 2488? If it was recent, it may already have a 160 in it. I bought one a year ago which still had 80GB written on the box, but the machine itself had a 160GB drive in it. I'd check the total partition sizes to see. If you've got an 80GB in there, here's a site I found with instructions:
Fuzzy's Dead Mouse - TASCAM 2488 Hard Drive Upgrade
The 2488 uses 3.5" form factor drives with EIDE connectors rather than SATA, which has been the standard for quite some time, so I have no idea if you'll be able to buy the drives anymore. You can likely buy EIDE drives on newegg, but there's no guarantee they'll work.