there could be a rather large performance boost, but i've always thought that the extra fps isn't worth the extra power needed to run a 2nd card along with the head that a second card will generate which also needs to be thought about, but it depends on what you already have and what you intend to use it for aswell and your screen size, but if you want a simple answer to your quesition, typicaly having 2 graphics cards will increase performance for gaming.