Overfilled hard drives may suffer fatal crashes. First thing is stop using that box until you make space on the hard drive. Best may be to remove data other than the Aperture Library, ideally at least 200 GB data or more. Then attach your fat new external drive (ideally eSATA, second choice FW800 rather than slow USB) and convert to a referenced-Masters Library with the Masters stored on the external drive, and backed up on one or more other external drives.
Personally I like quad-interface drives from OWC:
To add to Allen's comments, I'm surprised you've managed to get the drive that full without The Four Horsemen of the Data Apocalypse appearing and riding roughshod all over your data! Clean out your main disk asap!!!!
Being in a much smaller hard disk that yourself, I've already gone the external disk path. I had a look around and the one I got was the Seagate GoFlex Desk. They're a good price, but the coolest thing about them is that you can different bases with different interfaces. I bought the FW800 base and swap two externals onto it when needed - one for data and the other for the Time Machine. The Cunning Plan is to get the Thunderbolt base when it becomes available, so that I have an excuse to get a newer iMac (told you it was cunning, but my wife tells me I fail at logic...)
There's other good options out there also - WD have the Studio range with FW800 also, or you can buy an enclosure and do your own, there's RAID ones etc - just make sure they're FW800 for the best performance on your machine.
Are you running referenced or managed masters?
I'm running managed masters directly from my internal HD.
Firstly I went in and cleared 150 GB of non essential rubbish, so that should give me a little breathing space.
I've booked a gig with the geniuses at the mac store in london, so I'll run the migration process by them to make sure I don't incinerate a couple years of work..!
Thank you for the hard drive suggestions. I found this model online, its 2TB and Firewire 800. I've had an iomega before which I was pretty happy with. http://www.cancomuk.com/product/68461/34797/Iomega-eGo-Mac-2TB-External-Hard-Dri ve-%28-Firewire-800