Well, there's a bit of a conflict in your list of requirements.
For the most part, you're describing a workstation system, so you're looking in the right area. The problem you're going to run into is that with Nvidia, and they only support workstation class cards on workstation platforms. That boils down to GTX cards are not officially supported.
The E20 is not going to support auxillary powered video cards either, so you really should be looking at the S20 class most likely. If you're willing to move to a workstation class card (i.e. FX3800, FX4800, etc.), then the S20 might be your best bet.
Unfortunately, I don't really know anything about the Mercury Engine, so I can't speak to that compatibility.