I’m specing out a new Mac Pro also, I would go for the fastest processor and then I add RAM as I get extra money. You really need at least 3 GB of RAM to start off with (add 2-1GB sticks). Also if you plan to do a lot of graphics, motion, AE, combustion the extra $250 for the x1900 is probably a good thing to get.
I buy all my RAM from macsales.com. They now sell the same sticks as Apple for about 1/2 the price.
One correction to an above post, DVCProHD is very CPU intensive, but needs less drive through put compared to Uncompressed (even SD uncompressed). The stronger the codec the more processor needed.