Plenty of options left in the AM2/AM2+ platform area!
Here are a couple of boards that will support your processor. The Crosshair is a little over your budget but is an excellent board. The Biostar should do good as well but it's not what you'd call an extreme overclocker since it lacks MOSFET heatsinks, which I'd suggest you add before overclocking much.
Crosshair II $140
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131292
BIOSTAR TA790GXBE $95
http://www.newegg.com/Product/Product.aspx?Item=N82E16813138158
An
Athlon II X2 250 runs at 3.0 GHz stock and should overclock some from there, even with a cheap board. It should exceed your 6000+ performance by 5-15% and most likely will overclock better, though the cheaper board you'd have to buy for it (to stay within budget) would limit that to some extent ...