CGI (Computer Generated Imagery)

Hello!
I was wondering if anyone knows about CGI (Computer Generated Imagery), and if anyone could recommend me a technical specification of a machine to be built for creating CGI adverts (Preferably minimum requirements ad recommended requirements, there more info the better )
I have also posted in the software section of the forum about software specs for this type of system, if anyone knows about it well!!
Any advise would be greatly appreciated.
Many thanks in advance..

Thanks!
if possible, could you elaborate on why to chose the following (components you listed).. and which are more necessary etc.

As you might have guessed, I'm a bit of newb in this area and the more information i get the more useful!
I'm discarding the budget part of it for the moment, as I just want an open view of everything.. but of course budget will counts lots in the end

Also, I'm guessing these are mostly recommended requirements.. could you let me know of minimum requirements, and any in between if possible; would like to just compare around.. explanations as to why you would suggest them would help too.

Thanks again for help! greatly appreciated..

it won't necessarily be me using the 3D modeling software, just looking at the hardware option in order to get best performance versus cost.. but also the quality aspect of it, that's why i need few examples to compare and at least a brief explanation on the benefits/minuses of each, if possible.

Thanks for the responses by the way!!