Well personally I'd buy the drive and the enclosure separately. It will cost about the same as an all in one unit and you'll get your choice of capacity and drive brand. It will probably also be easier to upgrade the disk later.
Also, for maximum flexibility I'd suggest buying a combined USB2/e-sata enclosure. You can't go wrong with that combination and they're only slightly more expensive than a USB only enclosure. USB gives you the widest compatibility because almost all PCs have USB ports these days. And for those PCs that have an e-sata port, you'll be able to achieve the same transfer speed as an internal drive.