Advice please on processors and graphics cards

Ah, excellent questions.

AMD vs. Intel - I've been using AMD chips in the systems I build for the last 6 years. They have consistently offered the same or better performance for less money. That may change, as the new Core 2 chips by Intel are looking really excellent. I'd say that would be your best bet for the next few months, at least until AMD comes up with an answer.

P4 vs. Xeon /Athlon vs. Opteron - This is an issue that confuses many people. The difference between them is essentially this: the server version (Xeon, Opteron) are generally hand-picked because of the quality of the silicon wafer. Basically, this means that they are more reliable at higher clock speeds. They also typically carry more onboard cache. But is it useful for Dietrich's or CAD of any kind? Not really. Unless you are using your computer continuously, like a server, the reliablity issue is moot. The onboard cache might give you a small speed boost, but not enough to offset the prices on the server chips. Take a higher-end desktop chip (Athlon/P4/Core 2 Duo) if you have a choice.

Single vs. Dual Core - This is not really a choice. If you're buying a new, fast chip, it's going to be dual-core. It's the next step in the development of chips: they are experiencing dimishing returns by simple running at higher clock speeds. Instead, they are attempting to refine the architecture so that chips run faster while running at existing clock speeds.

ATI vs. nVidia - I've always preferred ATI cards, but there is no particular reason for that other than they are a Canadian company and produce great stuff. Either will do the job, and do it well. But more importantly, I don't know that you have to go with a workstation card necessarily (FireGL vs Quadro) because in fact Dietrich's is fairly light graphically...I have a higher end gaming card and OpenGL renders hardly ever move from the 50fps limit. I'd say a low end workstation card would easily cover your bases.

Tips - Don't forget the other components of your computer. Namely, the hard drive(s). Buy a 10k SATA drive or a SCSI drive. Even better, run multiple drives in RAID 1 - redundant. That way, if one drive fails, you seemlessly move onto the other drive while the failing drive is replaced. In my view, the 10k Raptor SATA drives would be plenty. Again, you're not running a server. Also, buy a quality power supply, like an Antec, Enermax, Zalman or PC Power and Cooling. Buy brand name RAM (preferably low latency) like Corsair, Kingston, OCZ or Crucial.

Finally, I'm going to buck the trend. I say, don't buy top of the line, or the most you can afford. Save yourself some money by buying the second or third down from the top model...you'll get 95% of the performance at 70% of the price. There is often an optimum price/performance point that I try to hit every time. By spending less now, you can afford to upgrade more often, which is really your best bet for having a consistently fast system.

Feel free to run system configurations by me anytime. I also custom build systems. The money I charge for building systems is often offset by the money saved by knowing where to source parts.

Sorry I didn't get back to you sooner. Looks like it should serve your needs pretty well.