Exabyte has some nice units that won't cost a whole lot comparatively. You'll want to account for data growth and you may want to also consider a leasing plan on your tape drive, since that will allow you to get something larger now and upgrade again in a couple years. Seems to be popular these days
The VXA-320 PacketLoader holds about 1.6TB uncompressed, which would give you ample size + the ability to grow. I'd think that anything lower than 1TB of size would be wasted, since you'll probably be pushing that storage size in the near future.
As far as software goes, there are lots of options. Some devices will include some sort of software. Some won't. Some will, but you won't want to use it!
Here's a paper I wrote on backup solutions for Mac OS X:
for backup software. it's been a great combo, fast, reliable and rather painless. the ait2 tapes average around 120gb or so per tape, and $50 approx per tape.