I'd actually be happier with a bigger Mac Pro. Some high end video cards take up two slots. Throw in a capture card that takes up two more slots and a RAID card and there isn't room for another high end video card.
I'd like to have 2 high end video cards (thats 4 slots), a RAID card ( 1 slot), a capture card (2 slots) and a Fiber card (1 slot). Thats 8 PCI-X slots.
Dual optical drives are a must a long with at least 4 internal hard drives.
Obviously a larger Mac Pro would...