If you check places like Sonnet and others, they offer processor upgrades for the PowerPC Macs prior to the new Intel Macs Apple offers.
As to whether you would by something from Dell or HP or whoever, that's not the case. The case is that neither of them offer anything substanially configurable over Apple.
If you build the system yourself, then you would know what CPU the motherboard takes. If a brand name company makes the computer, unless you open up the case and know where to look, then you're going to have a difficult time finding out who makes the motherboard (if it's an OEM motherboard) let alone trying to find out what CPUs it takes. We're not talking about retail motherboards here so it's not as easy.
As for Apple's configurations, I don't know what exactly you're referring to about the configurability of the build-to-order Mac Pro, but it's not unlike what I've seen on Dell's site for build-to-order.
http://store.apple.com/1-800-MY-APPL...CjuAJn6/2.?p=0
You have to compare apples to apples (so to speak), and comparing a DIY PC to what's available as a build-to-order option from a computer hardware manufacturer is not a fair comparison. Even the Dells, HPs, et al would seem less configurable than a self-built PC.
BTW, according to AnandTech, it is possible to swap out the CPUs with something else like the upcoming Cloverton CPUs (the Core 2 Duo Xeon). Just drop 'em in and you're off.
http://www.anandtech.com/mac/showdoc.aspx?i=2832&p=6