Keep in mind that Nehalem will require yet another motherboard (different socket) and DDR3 memory so anything you get to support an e8400 will not work with the next generation chips.
With that in mind, it really depends on whether or not the Opty is doing what you need it to do or not. If you are satisfied with performance right now, just wait a year or so and get Nehalem. If the Opty just isn't keeping up with your requirements, go for the e8400 and then wait about two years and get whatever is available after Nehalem. (For the second option you may be better with a quad instead of a dual if planning to use it for a couple of years.)