Im my opinion buying a retail pc is pricey, its good for things like customer support and warranties. when looking for a good editing pc its good to have top of the line hardware, at least a 3.0gHz with 1025 ram or more and lots of hdd space(250gig+ for multiple projects) . the hdd should have 16 mb cache because i think they make a crucial part of speed. External hdd are nice if your thinking of using the same data on different pcs but they get a little pricey. Buying an external case and a hdd for it is a smart way to go because this way you can switch hdds from case to case and it makes it an upgradable package. Also a fast respond monitor when looking in to the LCDs makes a really nice package.

consider building a pc from scratch there are lots of websites out there that will help you in that process.