The Duron 850s are actually at the same price (or more expensive) than the Athlon 850MHz right now, because the Duron 850MHz is brand new. Eventually, the price of the Duron will fall, but for now if you're going for 850MHz, you might as well get the Athlon, because the extra cache does make a performance difference, and your overclocking potential should be pretty much the same for both chips.
Oh, and both can be overclocked in the BIOS of the A7V. You can overclock using the Front Side Bus, or if you pencil your L1 bridges, you can also increase the multiplier. On some older A7Vs this is only available through DIP switches, although it seems that you would be getting a new one, so it should be in the BIOS.