For our group of designers, I've recently gone through this. We do large assemblies, part modeling, and drawings.
You want the fastest processor you can get. I spec'd the Xeon E5-1620 3.6 GHz (i think a 3.7 GHz processor is now available). Creo only occasionally uses more than one core so I opted for the 4 core processor. You'll use the other cores for windows and if you do FEA, you may want to consider more cores or more physical processors (FEA is not a major factor in my groups computers).
I tested our big assemblies and established that we were max-ing our RAM so we spec'd out 16gig of RAM.
In Wildfire 4 I was able to do some video card testing and we couldn't find much of a performance difference between the Quadro 2000, 4000 and 5000 BUT supposedly CREO 2 better utilizes the higher end video. I have not had the opportunity to re-test video cards since moving to CREO 2. We ended up specifying the Quadro 4000.
I have a SSD drive and I think it helps but I can't verify that with numbers. I think our IT guy did some testing on it and he was the one who sold me on it.
If you have an opportunity to test systems, I would recommend it. We used Olaf Corten's (www.proesite.com) CREO benchmark to help us make decisions.
Good Luck