Yeah, mofficer raised a good point. Although data transfer rates for Firewire is pretty good, in theory it won't compare to a direct SATA or IDE connection, peticularly SATA-II, if you have the capability. You extra concern however with building your own external will be connecting to other PCs (if you require it) and additionally HDD cooling

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If you do decide to go for external, LaCie and Maxtor seem pretty popular - they are all we use at work and seem very reliable. Can be pricey though - we have a LaCie Bigger Disk (1Tb (1000Gb)) which set us back about $AU800, but has USB2.0, Firewire 400 & 800 connections, and additionally has it's own cooling.
In the end it really depends on why you want it - if you don't need to take only your HDD with you, you are way better off just sticking with an internal drive...but thats just my opinion.