Julian – don’t know whether it helps, but I’ve got the same Mac Pro as you (2009 Quad Core). Over the summer I put in a Sapphire AMD Radeon HD 7950 – £320 very well spent as FCPX really flies now. Unrendered timelines with 2 or 3 layers play perfectly (at 1080p 25), and the rendering is very fast. I was struggling before with an older card, with beach-balling and crashes, but the difference now is like chalk and cheese. The whole program just feels alive now. Just wondered whether that could be your bottleneck?
Martin Phillips Freelance Cameraman / Producer / Editor
Chesham, Buckinghamshire, UK. http://www.thevideodepartment.co.uk