Lutfij :
RAM - You'll need a good amount of it. CPU - you could get an AMD x4 955BE but the i3-2100 easily trumps that...trust me, this is coming from a student of architecture using all kinds of rendering apps.
Firepro or quatro's are specially designed to take/share the load off the cpu. Will set you back a few quids but is worth the extra cash dropping.
Just remember, spending on a Gaming GPU like the ati HD 4xxx, 5xxx or 6xxx series is a total waste of money. They are built only for gaming. Using them for 3D work will only utilize a fraction of the gpu you have.
CUDA enabled GPU's from Nvidia are a little better on the otherhand.
Here's why
Actually as far as my understanding goes
Quatro and FirePro are actually same core as the gaming GPUS
but have a different set of drivers optimized for OpenGL which is what most renderers rely on
You can actually run some Quatro/Firepro drivers on the equilavent gaming CPU core
Nvidia has CUDA and I believe Open CL
while
AMD/ATI has ATI Stream and OpenCL
which are both ways of offloading parallel optimized code to the GPU
For example I use CyberLink Power Director 9 which uses ATI Stream and the GPU shares
the encoding with the CPU
DVD Fab will also work with ATI Stream
Nvidia CUDA does have more apps that take advantage of it than AMD Stream
CUDA has more software development that ATI Stream
due to NVidia focusing earlier than AMD on this tech
Hopefully AMD and Nvidia will agree on OpenCL which is an open source API project
and this way their will be a mutual standard reached
Really AMD and Nvidia using competing standards for GPU offload processing is actually hurting both of them
refer to
http://www.amd.com/US/PRODUCTS/TECHNOLOGIES/STREAM-TECHNOLOGY/Pages/stream-technology.aspx