The Quadro will perform almost identical to the 7900 GT in real world practice.
The CPU doesn't matter a lot except in rendering- I mean, what does AC use it for most of the time? 2d apps can run on the oldest video cards out there almost (10 years old). The difference between the two you asked about are almost unperceptible except a few times a day for specialized applications for specialized professionals (of which we often are here). However, if you're REALLY going to be doing a LOT of rendering, spending money on a FEW OLDER computers and linking them up in a network will be cheaper than buying one big high-numbers crunching rig. I don't recall however if C4D does network rendering, so that FOR SURE would be important to know in advance. However, 3-or-5 older pIII's, stripped down (no monitors needed, just PSU, HD and fans with a network (NIC) card or built-in and a bunch of old memory) will rock for rendering using most commercial renderers out there, vs. one overly taxed giant expensive top-model computer. The giant firms out there (IE NSA and intelligence firms) use networked machines to get things done. Now they can ALSO afford top model machines, but even if they couldn't they'd overcome the issues simply with mass number crunching on different smaller and slower machines vs a rival firm that was using just a few very powerful machines.
I mean that is what the whole multi-core design is about- removing the NIC and delays between more than one PC/CPU... so literally they put them on the same "core" (die)!
However, you choose well. It ought to last you a few years before you'll think about upgrading.
For those of you reading this and taking notes on the laptop MXM upgradeable GPU slot, the power requirements change or the AC adaptor often if you put in a beefier GPU, so factor that into an upgrade if you're planning on one someday- you'll need to buy a beefier AC adaptor and even the top battery (9-12 cell, for instance, from a 6-8 cell) that can fit the machine!!!!
You'll also need to know the power capacitors on your motherboard model (head over to the forums at notebookreviews or something similar to find threads for your model) before you plug in a GPU that will cook your chips or keep your laptop from even booting until a lower and slower GPU is reinstalled.
