No, it's not enough to have 2 X16 PCIe slots, since some mobos can only support Crossfire and some support SLI tech. Newer mobos however support both, but it's better to check the specs before you pick up something.
What you need to check with the newer mobos is whether both slots function @16x when populated or @8x when both are populated, any of the 2 combinations will do, either both @16x or both @8x, but, what you need to beware of are, mobos that support one x16 and one x4 when both are populated. Those are the mobos you don't need to waste money on.