Are Quadros really necessary for 3D design? I know gamer cards work more than well enough for 2D design, but I'm not familiar with 3D.
No they are not a requirement. If you are someone that can build your own pc then you are someone that does not need a professional card. The main benefit with professional cards is the support. If you are using a pro card and the software isn't displaying something properly then you can call up the software vendor and they will try to resolve the issue. If you are using a gaming card then some vendors will use the gaming card as the reason why the software isn't performing properly. That is happening less and less now though as more people are using gaming cards for pro work. If you know about drivers, and how to troubleshoot pc problems then using a gaming card is fine.
From Autodesk themselves.
Important: Although Autodesk tested the NVIDIA GeForce and ATI Radeon consumer
graphics cards, it is Autodesk, NVIDIA, and AMD policy to only recommend and support
the professional NVIDIA Quadro, ATI FirePro, and ATI FireGL graphics family cards.
Consumer Graphics Cards Caveats & Limitations
There are currently no known Consumer graphic cards caveats or limitations related to
the operating systems supported by the Autodesk 3ds Max 2011 & Autodesk 3ds Max
Design 2011 for Microsoft Windows software product releases.
If you look at the full compatibility list for apple and Maya they list the radeon gaming cards as the cards to use because that is what the Mac comes with.
The big thing with the video card you pick is onboard ram. You want as much as you can get on the card. 3d applications tend to use very large textures that often can consume 30MB for one texture. Have 20 of those in a scene and you can see where the ram goes.
Video cards did not figure into rendering until really about 2 months ago. With the release of their 2011 software , autodesk has added a new renderer called quicksilver. It allows one to render scenes with the gpu with a few limitations. Some shading and other effects are not supported , but it can still turn out renders that are very good much faster than the cpu.
So for the OP, it really is not hard to build a good pc for 3d work. There really is not anything special you need to use like a specific motherboard.
For under $1400 this is a system that will have no problems running any 3d software out there. The memory is 4GB to keep the start up cost low and you still have 2 slots free if you need more you can add it later .