Like Arcanedeath said above, all firewire enclosure allow for daisy-chaining devices. Otherwise, if you go with USB enclosures, you'll likely need a USB hub.
Still, the best advice is to copy or move all the data from the old, small drives, onto a new large drive. You'll save a whole lot more electricity and have quicker random access to your data. Especially with the AT crowd, it's not unusual to a stack of old hard drives on a shelf, or in a closet.