Hi Will ...
Can't answer question about multiple cards ... I only have a 9800 GT ... pretty basic but it does have CUDA and it works great for me.
I don't see any artifacts on the one rendering I have done since loading the new version. I really like being able to change the character of the materials on the fly while the rendering is happening.
I have a lot to learn with this product but what I see looks good so far.
The biggest problem I had was trying to find out how and where to download the upgrade. Finally realized it was the CUDA download ... I thought that was just a driver update.
cheers
eric