I would buy this
http://store.apple.com/us/product/G0LG1LL/A?mco=MTk3NjkyMjE
pull the cpus sell them on ebay after you put in your 3.4 cpus got to be the easiest way. kind of what i did with my quad 2.8 to hex 3.2 mod.
I second this. Back in the 2006 days, I bought 2.4GHz CPUs and BSEL OC'd them to 3.0GHZ 8-core.
My 2009 2.66 now has the W3580 3.33GHz, and 24GB (3x8GB) ram. Neither of which were purchased form Apple. I then use the Areca 1880ix-12 RAID card, and it blows the Apple raid card out of the water with my SSDs. Just found a great price on a gently used '09 Pro and went to town.
I've built a few from scratch, and aside from the experience, the cost/effort wasn't feasible.