Okay first heres the thing: 2x 5770 = 1X 5870, which makes it $100 cheaper. However, the 5750 is not an upgrade over your 4870 as a single card. The 5770 performs on par with the 4870. What would you rather have higher performance or a single card. That question is up to you. If it's a single card go for the 5830 and above. If its two cards outperforming a mainstream single card go for 2x 5770s.