I don't live in the US and I didn't find the ASRock 870 EXTREME3 in our local store's list in here. So based on the 2 boards you suggested, Asrock 880GM-LE is the cheaper one so I think I'll go with it.
Is the Asrock 880GM-LE good for cpu overclocking? And yes, I like to do crossfire in the future but I think the msi 770-g45 is the only one capable within my budget. It says that it has 2 PCI-E, one x16 operation and the other one x4 operation. Will a x16 and a x4 be good for crossfire? If not then for now I'll go with the Asrock 880GM-LE.
Asrock boards in general are pretty good for overclocking, but I find that asus, gigabyte, and msi boards are usually better in terms of stability. Overclocking will come down to your particular chip as well as the cooler you are using. If you are going to oc you are going to want an aftermarket heatsink. I don't know what you have available, so a list of what you can get your hands on would be nice. As long as you aren't going for some crazy overclock then the asrock board should be fine. x16/x4 isn't preferable for cf, but swapping motherboards is a total pain and I would recommend that you spend a little bit more for a board that can do at least x8/x8.