I've spec'd out CAD workstations for about 4 years now, mostly running Solidworks, backed up by Autocad for legacy drawings.
We've pretty much stuck to Dell Precision Workstations for the whole time. They're pretty reliable, but have some quirks sometimes. We've lost power supplies, hard drives and noisy fans on the graphics cards.
I can say without any hesitation whatsoever that an nVidia Quadro card is hand's down a good choice. The higher the better.
Here's the system I'm running now at work
Dell Precision 390, 2gig ram, Core 2 Extreme 6800, Quadro 3500, 10k rpm hard drives, GigE NIC, Windows XP Pro (32bit), 3gig switch set.
Solidworks is highly dependent on OpenGL capability. The Quadros are just about second to none with it.
Memory is also key. I would go with 3 (or more) if you can. At least 3. I wish I had 3 instead of 2, especially if you're going to do any analysis.
As far as processor, go with the fastest you can afford, because you will use it. I can't comment on quad core because I don't have one, but from what I get from the official Solidworks forums, it's not the be all end all because SW can't use it except in a few areas (Photoworks, Cosmos Analysis).
CAD tends to be a linear process, rebuilding down the tree (feature history), so it can only process pretty much one thing at a time.
That said, it's real nice to have something running in the background (IGES transation, etc), and check email or do some file management at the same time. This is where at least a dual core is nice. When we went to the Core 2 on our latest machines, we came from a P4 3.4gig, and I immediately noticed a big difference.
A fast hard drive is nice, but not a necessity. We run mostly from our network (BOO!) so I get a bad hit anyway. Try opening a 10gig assembly from a network vs your hard drive. Night and day!
I can't really comment on the requirements on other programs like ProE or Catia, only to say it's probably very similar to Solidworks.
Autocad can use the processor speed, but I don't think it needs the graphics power the 3d packages do. Unless you're using Inventor, which is a different animal.
If you're doing a Solidworks install, it's absolutely imperative you disable antivirus when installing!!!
Here's a tip: go to ecost.com. They carry bargain basement quadro cards. I got a brand new pny quadro FX1000 (2+yr old tech) for $99!! This was to replace a Dell one that the fan got noisy.