Good news and bad news in the Ultimate Linux Box department. First of all, the good news. Price War! The classic price curve inflection between “hot” and “almost hot” CPUs is rounding out. The economy may be in /dev/crapper, but hardware vendors' loss is your gain. And as I write this, RAM prices are insanely low, too—1GB is the new 64MB.
The bad news is what do you need an Ultimate Linux Box for these days, anyway? A midrange laptop is more computer than most people need, and you can put together a perfectly good box for under a grand. Why blight the Earth with a room-heating, hover craft-sounding, credit-card-maxing monstrosity?
First of all, you can never have too many web servers. Yes, you have an official process where new pages and programs get developed, then deployed on a virtual host on the staging server for in-house testing, then move to the live server when they're approved. But that's for routine stuff. If you're doing something entirely new, and want to get input from possible users, nothing beats having a “scratch” server environment.
And if you keep an inactive but ready-to-go copy of your main web server setup on your desktop box, you can be back up quickly if your main server gets cracked. Do any necessary upgrades, start the copy and that major security compromise only costs you a few minutes of actual fix time. You have the compromised box, captured intact but off the network, to study and document at your leisure.
Anyone who's responsible for an important site can make the case for having a server in reserve, and that box, rounded out with a few extra parts, might as well earn its keep as a high-end desktop.