[JFreak]

I think your best bet would be to wait until the next generation of TDM cards is released. The problem is, nobody knows how long you would have to wait. That is the cheapest option, and the most rewarding one if the new cards are coming sooner rather than later.

But if you really need to begin using that MacPro right now I think your best option would be to buy the expansion chassis, because once you upgrade to the next generation you can always sell that chassis to get some of the money back. If you cross-grade to PCI-express cards now, then that money will be lost for good.