The 8320s are just 8350s that are binned at a lower clock speed due to testing, but for the most part you should be able to OC them as high as you could with an 8350. The 8370 is a newer version, with slighlty higher base & turbo clocks than the 8350, so I would assume it can go slightly higher.
Basically, if you're going to OC, for the price of the stock 8370 you can get an 8350 with an air cooler, or an 8320 with an inexpensive water cooler (or really good air cooler), so you can save a bit.