Really large hard drives and external enclosures

I’m looking to buy or build an external hard drive for my PC, primarily to use as a backup device.

Right now, a national computer retailer has the new Seagate 7200.8 400 GB drive on sale for $199. Insanely cheap.

To make one of these an external drive, it’s just add an enclosure with a Firewire interface, and off I go? Maybe. Some of the enclosures I’ve seen appear to have a 200 GB limit on drive size. Some say they support 200+, some say 300+. Haven’t seen any yet that specifically say they’ll support 400 GB.

What’s the deal with drive size limitations on these things, and other than buying a nice looking one that says 200+, 300+ or whatever and hoping it works, how can I know for sure that any given enclosure will handle that 400 GB drive?