- CPU: Sandy Bridge i5 or i7, maybe AMD Phenom II X6 (but Bulldozer is comming this summer)
- a solid motherboard for the choosen CPU
- RAM: 8 GB (2 x 4 GB modules) DDR3 - 1333 or 1600 MHz
- HDD: min. 500 GB, but the 1 TB HDDs are cheap, so ... And maybe a second HDD for backup (if you need speed - Raptors in RAID0 or SSDs, don't know, how many money you have)
- do you need 3D graphics horsepower? Then look for some good gaming cards (aka Geforce GT 560) or if you have money left one of the Quadro series (look at Autodesk website what graphic cards are recommended)
If not, the integrated Intel or AMD GPUs are solid and works good for most type of work (but now for playing the modern blockbuster/AA title games)
- a solid computer case with good ventilation
- 400+ (and up) Watt PSU (Enermax, Corsair etc.)
- If you do not have a monitor, you should buy min. a 22" LCD with wide viewing angel (most xPVA and xIPS monitors have 178/178). If there is money, go for 24" or bigger versions of these. Maybe two.
I don't know, what is your preference, how big monitors and workspace you need.