At the risk of embarrassing myself, I've been continually upgrading a P4-based computer for the last six years, replacing pieces as they fail, or as technology threatens to pass me by....
Intel Pentium 4 @ 2.8 GHz (Northwood, SL78Y, 130 nm)
Asus P4C800-E Deluxe (Intel 875P & 82801/ICH5R)
2 GB DDR (PC3200) in dual-channel mode
ATi HD 3650 @ 725 MHz (RV635, 55 nm, AGP 8x) with 512 MB DDR2
10 drives (3 optical drives, 5 hard drives w/1.1 TB capacity, 1 1.44 MB floppy drive w/5-in-1 flash reader, 1 100 MB Zip drive)
Windows Vista (32-bit)
For anything processor-limited, it will get crushed (especially when pitting its primitive HyperThreading against modern multicore systems in multithreaded tasks). But the rest of the computer is reasonably competitive with modern systems. (Stripped down, the system can boot at up to 3.7 GHz, but it's definitely not stable at that speed.)
But despite that, it can handle just about anything CAD-related, at least in Pro/ENGINEER Wildfire 4.0. (However, Inventor 2009 just makes it angry.) To put this in perspective, I've got 5 FIRST robots open in Pro/E, 4 of which are complex sheetmetal designs with hundreds of parts, with 3 being native Pro/E assemblies and 2 being Pro/E assemblies composed of STEP import features. It's using about 727 MB of RAM, and all of the windows are responding normally, and the views render cleanly and animate at a full-motion framerate. In Inventor, I have one robot open; it's using 763 MB on its own, and the views are sluggish, normal view manipulation is occuring at around 10 frames per second, and the UI isn't responding properly.
When it comes to CAD in Pro/E my computer shows its age only when regenerating or doing things like structural and thermal analyses (which are processor-bound). The video subsystem is relatively modern, and plenty fast enough for CAD work (though the drivers are probably suboptimal). The main hard drive is fast and reasonably large (a 596 GB, 7 200 rev/min unit). More RAM would be a convenience for multitasking, but isn't a big deal here; in non-CAD use, my RAM usage is often around 1.8 GB—or exactly where it ought to be.
Despite that, I'm going to have to replace this computer reasonably soon, mainly to maintain my credibility, but also because I'm leaning toward something that's actually portable. At the moment, I'm looking out for the mobile Core i7s that are due in late September.