Ultimate CPU upgrade for Mac Pro 5,1 2 x 2.4GHz Quard-core

I'm having a go at modding my existing Mac Pro until the jury comes in on the 2013 model.

While I'm reading the 'All We Know About Maximizing CPU Related Performance' thread started by Tutor, I thought I'd start another thread to discuss the best options.

I'm looking for the ULTIMATE CPU upgrade. Any advice much appreciated.

Thanks :)