That thread is awesome, but it's also confusing and was started back during the Eclair days, but well worth a read to see the some suggested profiles in the app I'm going to suggest you get.
So, to wit...
SetCPU is the best one for overclocking. Period.
I'm not kidding. Don't even look elsewhere. Just go plunk down the $3 and smile.
As for how high you can go, your kernel supports 800MHz (so you can play Flash video), but you can flash kernels (as long as you have a custom recovery image) to go even higher and lower the voltages to save some battery life at the same time. As for the top speed, that depends on your Droid. Some choke at 800MHz, others can be pushed to 1300 (though those are RARE and generate a LOT of heat).