There are usually some decent reviews of hardware like this in Cadence Magazine. If you use ACAD, I would strongly suggest you get a subscription. I would get all the processor you can afford. The amount of ram you ll need is dependant upon your operating system, and whether you're networked or not. I would think 128 MB at the very least.
Video cards are a personal preference.
Most of the manufacturers have prepackaged CAD workstations, that have decent setups, i.e., Dell, Micron, Gateway, etc. You certainly can't go wrong with a Dell, but with the wide variety of uses you have planned, it may be wise to consider more than one computer. I have four different computers. I don't actually need all four, but each is configured for specific applications, along with some ability for backing up another in case of the inevitable. Luckily, it's quite easy to purchase serviceable used computers at rock bottom prices these days, so you may come out better price-wise to go that route.