Once installed MorphOS you can remove nvidia card.

Actually on my setup (g5 quad core) I have swapped an ati x1900gt (pc bios) into the first 16x pcie slot and moved nvidia 6600 two step upper into 8x pcie slot.
I have also two pcie ( one wifi atheros and one realtek lan cards) interleaved in remaining two slots.
All runs very well under osx/linux and morphos