I'm aware that there were topics on the old forums about this but I never took the time to read them since I wasn't considering multiple GPUs.

So what exactly are the Pros and cons of Nvidia SLI and and ATI Crossfire?
How much do they improve performance graphics wise?
How about crunching wise?
For crunching is it better to use this feature or individually?
How will boinc see the cards if you run this feature?

Also taking the fact I do some gaming on Cassandra would it be better to use separate cards or to join them?