XPS-8500 - largest size hard drive I can add internally

Hi, I have an XPS-8500.  It's got the Dell standard 2 TB hard drive, Win 7 Home Ultimate and 24 GB of RAM.  It's been a great machine but I'd really like more storage space.  I contemplated adding a Western Digital Caviar Black 2 GB but then I wondered how big I can go.  After all, it's only got two drive bays so I may as well not waste that last bay.

What size drive can I add?  Can I go to 4 TB?  If so, what ones have worked well?