In our office we run AutoCAD and Revit. For AutoCAD we get by just fine with 4GB of ram and 32-bit versions of Windows. For Revit we have had to switch to 8GB of ram and 64-bit Windows 7 in order to work with some of the larger models that our clients send to us. You will have to decide if solidworks needs 4GB or not. I personally would not go with a 64-bit OS unless my work demanded it. I have too many programs that will not run under the 64-bit OS.
I run more AutoCAD than Revit. I have a 3 year old machine that I spec'd with a dual core E8500 processor at 3.8GHz, Gigabyte motherboard, 4GB of ram, Windows XP 32-bit.
Our guys that run more Revit use machines that I built about 2 years ago, with i5-750 processors at about 3.5GHz, 8GB of ram, Gigabyte motherboards, Windows 7 64-bit.
We run mostly ATI FireGL video cards, with one Nvidia Quadro card. AutoCAD and Revit don't really need fancy video cards, but they need good video card DRIVERS, especially with Win7 64-bit, and we had to buy good cards in order to get sufficient drivers.
I will hopefully build myself a new CAD workstation in the next 12 months. If I built one today I would use an i5-2500k, at least 16GB of ram, Gigabyte motherboard, 120GB SSD for the system drive and multiple mechanical drives for secondary storage, Windows 7 64-bit, one of the video cards mentioned above. Unfortunately because I have a lot of software that won't run on the 64-bit OS, I would have to maintain a 32-bit machine running alongside the new one in order to run the older programs.
All of our CAD machines are now overclocked to 90% or more of their maximum speed. My machine has run 24/7 for 3 years solid, the machines that I built for the other guys have run 24/7 for over 2 years solid. Whatever I build in the future will be overclocked to at least 90% of what I can get from it. Needless to say I use one of the best aftermarket CPU coolers that I can find, and I add more fans to the case. Some of our machines with Antec 300 cases have 4 case fans, 1 cpu fan, 1 video card fan, and 1 psu fan. That's a lot of fans. My machine is right under my table and I don't hear it at all.
You'll note that my older machine has a dual core processor but our newer machines have a quad core. AutoCAD and Revit only use one core, but for the way we use our machines I have discovered that there area lot of benefits to more than 2 cores. I think a desktop workstation should have 4 cores (or more), only consider a dual core processor for a secretarial machine or a laptop where heat and battery life are considerations.