First of all, the upgrade to v4 is worth every dime. You’ll gain features and performance throughout the entire application.
I’m pretty sure that most resellers would be willing to cut you a deal when upgrading from MCAHD to v4 with a XD box. Actually, in Q4 of 2009, there was a pretty sweet bundle deal that gave you a Z400 with a Nitris DX when trading in your MCA.
MC 4 will surely run on the XW8400, though. Doing that myself. See the current system requirements.
Mojo DX is supported on the XW8400 (with the right Quadro FX card), but DNxHD capture is only supported on dual quad core versions of the XW8400. Not saying it will not work with dual-dual machines, but saying that it may not work.
It depends on the current offerings, but usually, you can either upgrade your current software from 2.8 to v4 (you can even do that online), or you can upgrade your software from 2.8 to v4 while adding a Mojo DX (or Nitris DX) with some discount. Then you can either add a Mac or HP to that (or keep your CPU). I’d tend to think that resellers would want to cut better deals on the larger packages.
Have you considered taking the Adrenaline HD to a support site? There might be some life in it.