No, the Asus A7V is not the best mobo for overclocking. If you have to get a new system this weekend, you'll want to get a MSI KT7 Pro2A, a Epox 8KTA2 or a classic Abit K7T. There are more possibilities, but those boards should be fine. You could also try looking for a board using the new KT133A chipset, but probably those aren't available yet.
About 800mhz Thunderbird versus Duron, both should overclock about the same. Reaching 1ghz is a good possibility with decent cooling. A Thunderbird of the same mhz will (arguably) be about 5 to 10% faster then the Duron. It's more expensive, of course.