We use fusionIO for tempdb too, beautiful! But why all the messing around with PCI expansion etc, if you need more space than you can squeeze into a box why not get a solid state SAN, then all you have to do is find enough space for HBA cards to give you enough bandwidth.
Now I don't have one of these so can't recommend them or comment on their suitability for SQL server but they do look the biz and I'd love to give one a test drive.