You CAN NOT SLI an Nvidia and an AMD/Ati card. You can SLi similar NVIDIA cards. AMD/Ati uses crossfire, so if your board does not support crossfire, you have to SLI your cards.
Since you already have a decent card, I would just use your 5770 until it doesn't suit your needs. However, once you play a game that maxes your 5770, your AM2+ CPU will probably be maxed too.