I am Mac OS X 10.7 Lion user. I want keep Mac OS X Lion, but I want get Mac OS X Snow Leopard, because I need Rossetta to continue working with my games. Can I keep Mac OS X Lion and Mac OS X Snow Leopard same time? Will dual-boot work between Lion and Snow Leopard? Is it OK to downgrade to Snow Leopard? But startup I will use OS X Lion, but when I something want to, I will switch to OS X Snow Leopard and switch back.

You can dual-boot as long as you keep Lion and Snow Leopard in separate partitions. You can also use virtualization software like Parallels if you don't want to repartition your drive.