I find this question kind of hard to answer.
Many people may indeed have Duron 600's that are capable of running at 1ghz, but the rest of there system, or poor knowledge is preventing this. Take a look at the CPU database at overclockers.com and you will see what I mean. Some users say there CPU will not overclock because it's mulitplier locked, some say 650 is the most they can get. Also, how many are running higher FSB, or +33mhz memory settings, yet only have cheap PC100 or worst yet PC66 ram? Also, many video cards are unstable if the bus is overclocked.
Any of these things can prevent someone from getting 900+mhz with there 600's. You really need to have ALL things equal to see the percentage of 600's that reach 900.
Last of all, 900mhz or even 1ghz are not the "magic numbers" if you use 100mhz FSB to get there. Higher FSB means much more than higher muliplier. For example, a CPU running at 896 (112*8) runs faster than a 1ghz cpu using 100*10. Yet, even though it's faster than a 1ghz, it fails your question of getting 900mhz. Perhaps a better question would be "how many 600's get 900 or better performance". When I see most posts saying how they can or can't get a certain speed, and they are at round numbers like 850, 900 or 950, it seems most are not overclocking the correct way to begin with. USE FSB along with multilpier!!!!!