A much cheaper alternative to the Wimberley heads is the Manfrotto 393 (Bogen 3421 in the States). I saw a fellow recently using one with a Nikon 600mm, and the performance and smoothness and stability of operation were excellent. Worked well with my Sigma 500 f/4.5, also.
Here is a link to a brief review with pictures:
http://www.digiscoped.com/manfrotto701RC2.html#anchor_21