There's a really cool one that's essentially just an external housing for any 3.5" HDD you want. It looks just like the mini and stacks underneath it. There's an even fancier one that gives you more ports too!
As seen here:
Mac mini memory, Mac mini hard drive, and Mac mini DVD upgrades at OtherWorldComputing.com
Agreed.Whichever one you choose, use the firewire connection, clone your internal drive to it and use it as your boot drive. The slow hard drives in minis (4300 rpm in first gens) is the biggest performance bottleneck. You will be amazed by the performance increase by moving to a 7200 rpm drive.