for the CPU
I would suggest get one with high GHz number. a quadcore is good. But I personally don't think you need hyperthreading (basically treating 4 cores as if they were 8) since Revit isn't good in parallel-computing.
every time i update a family etc. the CPU only uses 25% (1 core). So rahter spend your money on GHz instead of multithreading. Unless money is no object, then do both. This single thread behavior is one of the things I hope to be better in 2014, But I wouldn't hold my breath. It seems many IT fols spend a lot of money on server hardware workstations with even multiple CPUs, while most of Revit still works on one core only.
i have a 3.5GHz quadcaore at work and I'd rather have a 4GHz dualcore for Revit for this exact reason.
16GB cost around $70, so this is a no brainer. If you had 6GB so far I doubt you need 24 or 32 GB. It really depends on the plattform, if you use a more expensive tripl-channel platform, then you go in 12GB, 24 GB etc. If you use a cheaper dual channel platform, you go in 16GB, 32 GB etc.
Autodesk says 8GB is MINIMUM, the same way Mocrosoft says 512MB is MINIMUM for windows 7. At minimum double the minimum recommendation.
If you keep computers a long time, buy a board with 4 expansion slots so you can upgrade to 32GB if you ever need to. But at that time you likley want to repalce everything else anyway.
On graphics card I can't say much, but use one of the approved ones and if rendering is your main goal, the 2GB recommendation from above sounds good. Maybe someone more a graphics expert can tell. I'm not sure what rendering you do, but much can be done in the cloud by autodesk server if you have many renderings.
For harddrive an SSD (Samsung is most reliable, Intel too) is a must imho.
Except for the graphics card you can build a $800 Revit machine that will run circles around any of those Dell workstations for $3000.
