You did not say what you're backing up from. Is it an Apple server or some other? If Apple and you don't want to remove the tapes off location, Apple offers rather large and expandable multi hard drive storage solutions in it's Xserve line.
If you're backing up from something else, all teh big names like HP and IBM offer tape drives that certainly work well althoought they're not cheap. Yu just have to make sure that you have the right connections available. many are scsi.
An alternative is something like the LaCie 4 bay removable raid solutions that hold up to 2TB and are very reasonable. They connect via ethernet, are Linux based and connect to just about any platform.
Using OSX 10.3.9 on a G4 & G5