How Do I Keep Cats Out Of My Back Yard?

This is a carryover from where another question lead…

I want to adopt a cat or two from the humane society to help keep the snake population down at my house. The problem is that I have two bullmastiffs in the backyard that hate cats. There is a 6ft. wooden privacy fence around the backyard. I know cats can jump this, but if they have free reign of the front yard (about 1 acre) and access to bed, food, and water in our garage through a cat door, will they likely jump into the fence w/ my dogs? I thought of adopting adult cats who’ve never been inside cats so they don’t try to come inside or play with my dogs.

Are there any cat owners here on Fluther that know if this will work? Will cats even hunt small snakes? Do most cats have sense enough to not jump into a backyard with large dogs? I’m at my wits end w/ what to do about the snakes… in a year and half we’ve had 3 try to come into our garage!