ATI-based mobo isn't compatible with a nVidia graphics card?

I posted here recently about a problem I was having: on my new MSI
RS480M2-IL motherboard (based on the ATI Xpress 200 chipset), my new
MSI NX6600-TD256E (GeForce 6600) video card is hanging after a few
minutes of playing a 3D game. I contacted MSI tech support about the
problem, and this was their response:

"This is normal since the on board ATI chipset does not like any Nvidia
chipset based video cards. You can use ATI video cards (PCI-E) or
change to different mb with different chipset. This is an unofficial
problem for ATI chipset on the mb and not caused by MSI."

My question is: is this tech blowing smoke, or is this a real
incompatibility? Do ATI-based motherboards really have a known problem
with nVidia-based graphics cards?