PCI 3.0 is backwards compatible with 2.0 - No issues there.
The biggest question really is do you need SLI support? Motherboard without SLI support are generally significantly cheaper than motherboards with SLI support. ASUS p8z68-v LE is a great choice for a non-SLI ready motherboard.
ASUS P8Z68-V LE LGA 1155 $132http://www.newegg.com/Product/Product.aspx?Item=N82E16813131773
Extreme3 Gen3 is a popular budget choice, but by no means the best.
This would probably be my choice, keeping in mind the $160 budget:
GIGABYTE GA-Z68X-UD3H-B3 $160http://www.newegg.com/Product/Product.aspx?Item=N82E16813128498
A little higher $180 and id take the
ASUS P8Z68-V/GEN3 LGA 1155 $180http://www.newegg.com/Product/Product.aspx?Item=N82E16813131792