Good suggestions by warwick.
I would recommend the ATTO controller combined with the HP 1760 as well. Or if you go with the Quantum LTO-4HH (I have this one myself) I recommend the LSI SAS-3442E HBA, which is also available in a bundled kit from Quantum. Thus far, the Quantum drive does not always play nice with other HBA's, like the ATTO 380.
The desktop Quantum kit with SAS HBA, blank tape, cleaning tape and 1.5M SAS cable can be had for under $2300 USD. If you're within the USA, Provantage.com has some of the best prices on various tape systems. I believe they have the HP units too. I helped someone buy and install the HP 1760 about 2 months ago. We bought direct from HP's online store -- they had special discounts at the time that were better than any other vendor we could find, even better than TechServe, etc..
DELL also has some really nice tape systems too, but I don't know of anyone who has tried them on a Mac. Tandberg's tape systems also work well with Mac. They recommend ATempo software and Tandberg has some of the best offerings for low cost multi-tape systems.