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Each of the kernel "makers" generally have a range of top end speeds, with 5 to 7 slots below. And, they usually have low voltage, medium voltage and/or stock/high. It is generally accepted to check the OP of the rom you are using to see which maker is the best choice. Once you choose a maker, then you have to decide on voltage. WORDS OF WARNING, you can get stuck at boot if the voltage is not right for your phone, or if the top speed is too fast for your phone. You can also get forces closes and other problems if things do not work well for YOUR phone.
If you do not perform a nandroid (CW or SP) recovery before try a new kernel, you are asking for trouble. There are no definatives here, as each phone will behave differently as each and every CPU will be slightly unique. I guess I should correct myself as one fairly constant feature is the stock droid running at 550 or 600 has some room to go up. Most of the Rom makers will include a 600, or 800 as these have been proven to run fine on most phones. Occasionally you will see a rom come with a 1000, but I think most have backed off this.
And, you will either need the Setcpu app or a rom script to take advantage of higher speeds. Your best bet is to read through the op and all threads of the rom you are running, however painful it may be, to see what the folks running that rom have to say.