The 8xx Opterons are very expensive but also incredible because of the integrated memory controllers and the multiple HT buses they use to communicate with each other and the outside world REALLY fast.
There are no 4xx Opterons that I am aware of. Quad and 8way Opteron boards use Opteron 8xx CPUs and they are still socket 940 just like the model 2xx Dual Opterons.
The Tyan K8QW S4881 supports up to 4 Opteron 8xx CPUs and has PCI-Express so I suppose it would work for gaming in theory.
Some Dual Opteron boards, like the Tyan K8WE S2877ANRF, also support PCI-Express so they would work for gaming as well.