I don't know how good parallelization in Autodesk Inventor is -- but I'd guess for rendering tasks you'd benefit from multiple cores. For that reason, you might even want a quad core.
The new AMD Athlon II X4 620 [ newegg.com ] might be a good fit for you (2.6 GHz, Quad-Core). The reviews indicate people overclocking well above 3.0 GHz.
I won't try to pick out a motherboard for ...



