Ahhh the $3,000,000.00 question. I think most these days have seen how cost effective it can be to build your own PC, much like getting into recording, building a PC requires some reading, and basic comprehension skills, but it can be well worth the effort.
On a spec. related basis, there are still variables to consider. For CPU you want to look at whether it's 64bit or not, if it's 64 bit, you should start comparing L1 and L2 cache sizes before you start comparing core speeds. If it's your typical CPU clock speed will be more of a factor, but still L1 and L2 cache sizes will make the difference. Check out
www.newegg.com for parts info.
RAM - I'd say at the prices RAM has sunked to over the past year or so, 2Ghz of RAM is very doable, and well worth it. Be in 2x1G sticks or 4x512mb sticks, of course you can still go higher than that.
Harddrives - You want to look at the RPM (rotations per minute) Most common IDE drives these days are around 7200 rpm, there are some 10,000 RPM drives out there as well, the faster the better, but 7200 works decently enough. There are also SATA drives, which are faster by design that IDE drives. Do some reading and take your pick.
External Harddrives are more convenience from what I've read,seen,heard. If using multiple PCs or transfering data between locations, it beats copying to multiple CDs or lugging your whole PC with you. If you don't intend to transport regularly or lock away your data in a fire safe, it isn't needed imo.
We'll leave it at that for now, keep the Qs coming, I'll get back to ya as I have time.