Here's the basic deal with Intel overclocking. Most all of the current Intels of whatever speed, as long as the are fcpga chips, will do at least 850. The cC0 latest steppings will do 933 often. Some of course will do a ghz. But not more than about half, according to ed over at overclockers.com
It doesn't make much difference what speed intel you buy as to how fast it will theoretically overclock to. It's the luck fo the draw. Except of course you have a guarantee that it will run at whatever its marked at. What makes much more difference is that you have the lastest stepping, which is cC0. Stepping is basically a manufacturing run. (They tweak to improve their yields/performance between steppings, as they accumate manufacturing experience.)
The only way of overclocking Intels is by increading the speed at which the memory runs, the fsb -- because all Intel CPU's have locked multipliers (unlike TBirds). The easiest and most stable way to do this, which works on virtually all fcpga motherboards, is to buy a chip sold as a 100mhz chip, and run it at 133, if you are talking about PIII's ,or buy a Celeron fcpga rated at 66, and run it at 100 fsb.
The classic move then is to buy SLOWER marked intels, because they are not only cheaper, they are also more of a sure thing to work without problems when you do the one big jump overclock from 100 to 133, or 66 to 100.
The classic PIII's to get for this are the 600E (NOT EB), the 650, or the 700. The classic celerons are the 533 fcpga, 566 or the 600. THe lower speeds are safer, since it is virtually certain they will run when increased in speed by 1.333x by running them at 133 fsb. The higher speeds have a higher potential top overclock, since you won't have to push the fsb as much over 133 for PIII's, or 100 for Celerons, to get the same ultimate overclock chip speed.
Note in the PIII's you DON't want the chips which are sold as 133mhz chips, e.g the 600EB, the the 667 or the 733 (or anything higher). All of the lower speed Celerons are sold as 66mhz chips, but there avoid the 500 and the 533 fpga -- cause they are build on an older manufacturing process and won't overclock worth squat, usually. Mastering these little bits of complexity is a large part of the basic overclocking art.
Once you have it running at the big jump in fsb, then you can try to increase the fsb to a little more than 100, if it is a Celeron, or a little more than 133, if it is a PIII -- so long as your motherboard supports small increment increases in the fsb. I.e., so long as it supports overclocking. Another trick is to increase the core voltage a bit going to your processor, if your mobo bios supports that. We're talking about 0.05v increments here, or less if possible. Read up on how much is too much for the particular processor. (I'm up on Athlons, not Intels at the moment.) You start to need bigger and better heatsinks/fans as you increase the voltage and your chip runs hotter. For basic cost effective overclocking don't push it too far. As soon as anything becomes at all unstable, back down a bit. There is also a tendency sometimes for a chip to run a little faster once it has been "burned in" at a close but a bit lower speed. If you aren't in it for the sport, you won't want to push it too far. You'll care more about stability.
That is overclocking of Intels in a nutshell.
Good luck.