If you want to know the minimum, I have seen a PTC guy doing a live demo in WF 5 running on a laptop using a celeron processor with 2 gb RAM and an intel video chip in Windows XP 32 bit. Of course, if you want to do some serious engineering...
64 bit OS is a must. Believe me, you don't want to go 32 bit when working with medium - large assemblies in Pro/E. Still, you can survive with XP Pro with the 4 GB switch but I suggest you go 64 bit with Windows XP or 7. It runs on Vista, but that OS alone would consume a big chunk of your hardware resources.
As for processor, get the best multi-core processor you can lay your hands of. Xeon class processors are great for desktop workstations and i7 are the way to go for laptops. The processor speed would depend on the size of your assemblies and specific requirements. Despite what PTC would say, I personally would go for at leastone dual core processor running at a minimum of 2.2 ghz. For large assemblies I'd use a 3.0 gb processor clock speed.
RAM - the more the better. 4 GB would do for my average users. 8 GB would be good with a 64 bit OS if you are planning on pro/mechanica simulations and/or working with large assemblies.
Hard disk depends on whether you are storing your files on a network server, but BY ALL MEANS GET A RAID CONFIGURATION. Having a second hard drive as a mirror would only cost a few extra dollars but it will save you lots of time, headaches and money in case of a disaster. Believe me, I've seen an engineer cry after losing 1 week worth of work because of a damaged hard drive caused by a power surge. I'd say a RAID array of 2 160 GB disks running at a minimum of 7200 RPMs would do. Go for 10,000 RPM disks if you can, it reflects in the response time.
Any Nvidia Quadro or ATI Fire with 512 MB DDR3 RAM would do for the average CAD user, but I'd reccomend 768 or 1 GB of dedicated video memory if possible. If budget is a constrain you may go for a gaming card with 1 GB DDR3 memory Nvidia or ATI card and it would be a lot cheaper while delivering a decent performance, but stick to Pro video cards as much as possible. Do not use embedded Intel video chips, they are simply not designed for this kind of work.
As for additional hardware, I personally favor the use of two flat panel monitors and a spaceball. The spaceball takes some getting used to, but after a week or two you will wonder how could you ever do CAD without it. The two monitors is something I consider a must.
I hope it helps.