The 6000+ is the best you'll be able to use in that motherboard. It's also still for sale at Newegg: http://www.newegg.com/Product/Product.aspx?Item=N82E16819103772
In my research, I found that the board only officially supports up to the 5000+ (I think). Someone at this website
, though, bought Newegg's 6000+ and had it working.
Be sure to update the BIOS before upgrading the CPU.