Do a lot of research. Seriously, you have a lot of options. You can get a complete unit and depend on the vendor to use a good chipset and good drive. You can get your own empty enclosure-selected on the basis of chipset and price-and your own bare drive-selected on the basis of capacity, brand, and price-and put them together. In those two big options you can go from getting very expensive crap to getting very affordable high-class hardware.
Here's an example. I just got a firewire/USB 2.0 combination enclosure. It's a Coolmax HD538 5 1/4" external enclosure (I had originally intended to put a tray-loading DVD burner in it), and it cost me
about $30 from Directron.com. It has an IDE interface, so any high-capacity IDE drive would be a good choice-like
this 750GB drive from Newegg for less than $160. Not counting possibly interesting rush shipping, you wind up with 3/4 of a terabyte of storage you can connect to in two different ways for about $200 and a little time on your part. I think that's a pretty good deal.