Wich mobo should i get?

Nearly all of the boards you get are going to require a bios flash before the 4100 will post. The boards aren't bad; many just don't come with the bios file that you'll need. You'll need to pick a vender that can do this for you, or get a sempron 140 (check for one used on craigslist) for around $25. Once you flash the bios, you should have no problems. Newegg won't flash the bios for you. http://www.newegg.com/Product/Product.aspx?Item=N82E16813131758
@o1die - correct, I had Tecmo34 run that story.

However, if the GA-990FXA-UD3 (rev 1.2) has BIOS FA, FB is minimum for any FX, you can for 'free' send the MOBO to Gigabyte (any MOBO manufacturer) and they'll flash it for free. Some high-end, though non-AM3, e.g. ASUS allow the BIOS to be flashed for without the need of CPU or RAM.