hey folks I am just asking people around on forums what my next purchase should be. I am thinking of purchasing the ATI 5870 graphics card, but the thing is I have XFX nforce 790i Ultra motherboard which is a SLI mother board. I am thinking of purchasing second 5870 later on as the prices will drop, but I am not sure if I can use crossfire on SLI board.
No, you cannot run Crossfire on an SLI board. The newer 1156/1366 Intel boards can do both, but the old NVidia chipsets only do SLI. Your only choices would be a new MB or a 5970 (Or NVidia's new offerings) to get more performance.
No CF in a SLI board. You could still opt to buy the 5870 for now. But by the time you decide to buy another one, you should also change your mobo.