As long as you're going with a quality manufacturer, the chipset is probably the key descision. Some AMD boards support either DDR2 or DDR3, so be careful about that. After that, you're looking for form factor (ATX vs. microATX), I/O (USB and SATA ports etc), and then the bells and whistles (heat pipes etc).
For the AMD processors, I like the AMD chipsets. The 700 and 800 parts all can support AM3 processors like your Phenom II, but if you're buying new, the 800 parts (870, 880G, 890FX, 890GX) are all newer. If you are using the on-board graphics, the 880G probably fits your 'medium price' requirement. If you're using a discrete card, the 890FX doesn't have the GPU in the chipset. You can put a discrete card into a 'G' board without a problem if the price is right... the onboard GPU is just 'unused'.
Gigabyte and Asus are my current favorites. I've used Asrock and MSI in the past.
The cheapest 890FX board I'd buy at Newegg is in the $170 neighborhood. For under $100, this one looked pretty nice (Asus 880G mATX with USB3): http://www.newegg.com/Product/Product.aspx?Item=N82E16813131660
or $100 after rebate for a full ATX: http://www.newegg.com/Product/Product.aspx?Item=N82E16813131668
Those are just a couple to look at, there are tons of options.
Check prices and reviews at a place like newegg... or put a board you're interested in here (or in motherboard section). You'll get feedback.