What Processor/Motherboard Should I consider for 3d /Video Development

Hello,

My current workstation specs:

AMD x 2 4200
4 GIG RAM
XP 64 bit
500 GB with 32MB cache

I would highly appreciate it if you you guys can help decide upon a motherboard that would server my needs. I am currently in process of putting a workstation together for my 3d and video development. My current workstation slows down when I open more than one applications at the same time such as:

Adobe Premier
Adobe After Effects
Studio MAX

I would like to stay with AMD and have looked into AMD Phenom X4 940 / 3.00GHz, but I am also considering Quad Core XEONs. I would like to have at least 8GB RAM to start with. I will be using this workstation for development and 3D rendering.

Appreciate your help.
Thank you.