I would suggest at least 12GB RAM if you intend to work with anything heavy Vue or other 3D applications. C4D I know is a beast and REALLY shines with memory just like most 64bit software.

Personally, I use a Mac Pro with dedicated duel-boot drives, one for Snow Leopard and the other for Windows 7 Ultimate 64bit. Right now I only have 10GB but will soon be getting a new 2010 Mac Pro when available with 24GB RAM and SSD drives. I plan to relegate my 2008 Mac Pro as a dedicated Render Node and make it a part of my render farm for my 3D software that allows multi-computer rendering.

In short, get the most you can afford if you plan on any detailed work.

Jeff

Development on: Mac Pro 2008, Duel-Boot OS - Snow Leopard 10.6.6 & Windows 7 Ultimate 64bit, 2 x 2.8 GHz Quad-Core Intel Xeon , 10GB 800 MHz DDR2 RAM, NVIDIA GeForce 8800GT.