I have the opportunity to build a computer for a variety of CAD applications (design work, animations, simulation, etc).  I'd like to get everyone's two cents on what to get in each category.  Say your top three picks (ideal, realistic, and cheap but still useful).  I'll start off using SW08, but should be upgrading to the most current release in the near future.

Processor (what's the difference between a xeon or a iCore?)

Graphics card (leaning towards nVidia Quadro FX3800)

RAM

Hard drives (a friend suggested a solid state drive for program files and a traditional one for data storage.  would this cause problems?)

OS (32-bit vs 64-bit, XP vs Vista vs Windows 7?)