You can configure HP's Z800 workstations with two 6-core Xeons, including some higher-spec CPU models than the ones Apple has available, and up to 192GB of RAM, 6 HD bays, way more graphics options, etc., for a system costing up to several tens of thousands of dollars.
I did one HP Z800 configuration similar to the Mac Pro 12-core, with the same CPU and RAM and slightly better graphics, and it came out over $1000 more expensive than the Mac Pro.
You can configure HP's Z800 workstations with two 6-core Xeons, including some higher-spec CPU models than the ones Apple has available, and up to 192GB of RAM, 6 HD bays, way more graphics options, etc., for a system costing up to several tens of thousands of dollars.
I did one HP Z800 configuration similar to the Mac Pro 12-core, with the same CPU and RAM and slightly better graphics, and it came out over $1000 more expensive than the Mac Pro.