From the results I have seen, I would be very tempted to buy a Duron 750... They seem to overclock to 1000Mhz+ quite often.
Also, it's about 1/2 the price of a 850... $46 I believe.
Of course (as I'm sure you know) this all depends on your case cooling, cpu cooling and luck of the draw in getting a good chip.
If you are a gamer, unless you have a GF2Ultra or a GF3, anything over 800Mhz is going to max out your videocard anyway. [worst case senerio, you get stuck with a Duron 750 - Say you have a GF2MX - this system will be so close to a [other worst case senerio] 850 Duron, that you wont notice a difference. AND you'll have an extra $40 in your pocket
I know it is hard to convince yourself that buying a slower chip to start with will be faster in the end... Thats what I said when I bought my Celeron 466... thing does 525 max... Tons of people have 366's @ 550.
Just think of all those Duron 600 @ 1000+ basterds
As you stated you dont know if it overclocks well, take a few days and look around the internet, do your homework... You will soon decide what CPU is best for your needs.