I would go still cheap on Motherboard,
you do not need Nvidia,, use onboard video chipset specially if the core is going tobe in closet.
Look for more pci slots on the motherboard.
its just my two cents.
I have to disagree with that I'm afraid; he wants to build a hybrid, and assuming you want the fancy UI2 and you don't want to be frigging around trying to make it work with unsupported hardware, then go nVidia, just to be certain. Unless you have had a positive experience with this specific setup, theteju?
When you come to convert your hybrid to a standalone core, you can remove the nVidia card if you so wish (and use it in another MD perhaps), but you'll probably only get UI1 out of the onboard graphics.
I agree on the PCI slots, I like to have at least 3.