[moved to hardware forum]
The absolute best quad core system, an i7-2600K overclocked to 5.0 GHz, 32 GB memory and two Samsung 840 Pro SSD's in raid0 for project disk with a GTX 680 comes out at rank # 20 for CS6, in the upper quartile range so it belongs to the top 25% of observations. Everything faster is only hexa-cores.
Everything with a Matrox MXO2 comes out way slower, especially on export to disk, even with hexa-cores. That is a Matrox anomaly. Matrox just slows down a system in comparison to a system without Matrox.
I'd say it depends on your price point. Premiere Pro gets great performance by taxing much of a system including cpu, gpu, drives, bus (motherboard), and memory.
If you are wanting to build a desktop system yourself for under $1700, then I'd say Z77 4-core would be excellent. You can edit AVCHD for sure, but you would be more limited for complex projects.
If you are willing to spend more, the x79 chipset and i7-3930k 6-core is an excellent way to go.
Search around this site for awhile and you will find various build lists with lots of passionate comments on component selection for a wide variety of price points.