From what I read (not from my own experience, heaven knows I wish that'd be possible ), it does not really matter which Duron you get if you are going to overclock it. All Durons reach something like 900 - 1100 Mhz, depending on whether you're lucky or not. A 800 Mhz Duron hasn't got a higher chance of reaching the max than a 600 Mhz one.
I've never heard the same about Thunderbirds, however.
On the price, I agree with ravedave. Here in Germany, that sweet spot would be at 750 Mhz for Durons or at 900 Mhz for TBs. But that's probably different in the US.