Having two graphics cards in at once are likely to cause more problems than anything. Graphics cards are power hungry, and depending on the type your old one might even consume more power than the new. If your power supply isn't designed to handle it you could have some problems on your hands, and could potentially even cause damage. When I first built my pc at microcenter I got too small of a power supply for the radeon 6970 (requires a 550 minimum, the pc-building "expert" at the store sold me a 520w high current gamer) Right off the bat, the fans on the card were at full blast like a lawnmower and the computer crashed every hour or so. So i ended up returning everything and getting better stuff. Lesson - dont trust those "experts" at computer stores, best buy, home depot, etc. In my experience they usually don't know diddly.
