if i'm not mistaken, Dell has some sort of contract or deal with Intel that says Intel will provide the mobos for all of Dell's systems. so not only do Dell systems use mobos with intel chipsets, but the actual mobos themselves are usually made by intel also. although intel makes some of the most stable and reliable boards out there, they do not have any OCing capabilities. usually mobos made by intel have a jumper on the board that allows the system to start in "maitenance mode" when rebooted. and from what i hear, this is the only way to adjust the FSB on an intel mobo, and therefore the only way to do so on a Dell PC.
OCing will void the warrantee with Dell as well as any warrantee you might have with Intel also. in fact if you change just about anything in your Dell system, such as a video card, it will void your warrantee. i had a Dell once for about 2 years, and to receive tech support from them, i couldnt tell them about any parts i might have changed over the two years...
like erub said, the multipliers on any intel CPU are locked, so the only way to OC an intel CPU is through the FSB...and any intel-made mobo severely limits your ability to even do that. not to mention the 800E is nt the greatest OCer, and the 800EB is even worse.