Best rack mounted Mental Ray Renderer

[ThE_JacO]

Any renderfarm suitable box will be fine for mray rendering.
Just look and shop for renderfarm/RPU boxes in general, there's nothing specific to one used for mray vs anything else remote processing is used for really, unless you buy into nVIDIA's GPU rendering pimping that is, which on a farm is absolute BS anyway.

[olson]

The cheapest route will be to build your nodes from scratch. Buying from a system builder like Boxx or Dell, HP is another way to go but will cost more for what you get. Somewhere in between would be bare bone systems from a manufacturer like Supermicro where you add the CPU, drives, and memory.

http://www.supermicro.com/products/system/1U/#dpXeonSeaburg

They also have Opteron based systems:

http://www.supermicro.com/Aplus/system/1U/#opteron2000

Depending on how many you need it might be worth it to look into blades instead of complete systems. Cheers!