I think there was a thread similar to this a few months ago...
I found it to be more fun (and slightly cheaper) to purchase the external enclosure separate from the drive. I think the downside to this is you forfeit the warranty that typically comes with prebuilt externals (OEM drives = 30 day return).
I like this because you can carefully pick the drive based on newegg reviews. For example, Seagate has always been associated with top quality, and my 300GB drive has served me well. However, when I went to get a 500GB for my external, it turned out that the current models had an unusually high failure rate. So, I went with a similar Western Digital that had better reviews, and it's still serving me well.
I personally don't care for warranties, since they are only somewhat useful if you use full-disk encryption. I don't like the idea of sending in a drive full of sensitive data.
