Just to throw a little curve ball into the discussion, I recently spent WAY too much time recently specifying a new AMD-based home-media-oriented system. While I've built many systems in the past, I wasn't aware of how good the AMD Lllano and Trinity quad core chips had gotten.
For under $400, using top-notch parts, you can build a general-purpose home machine or high-end home-theater system. The AMD A10-5800K, especially, I think has a real price/performance advantage right now. At least that is how I was leaning.
The on-board AMD GPUs add a LOT of great things, for only $20 more than a comparable FX chip, (like playing smooth 1080p HD video, transmitting uncompressed digital audio through HDMI, having excellent 2D graphics and decent 3D game performance).
Some people don't like ATI graphics on Linux, but I've used it for years and do not have a complaint. Yes...I know Nvidia is probably still better supported overall, but AMD/ATI has really come a long way.
Anyway, if you want absolute maximum CPU performance and want a discrete 3D graphic card, then AMD FX is good, but the Intel line is probably worth looking into as well. I was not that sold on picking an FX chip over the Intel i3 or i5, but I did feel the A series AMD GPUs were great bang for the buck.