No you can't. Dell's have a limited BIOS. I have heard that some of the newer very highend XPS model's have a very limited overclocking setting (you can push a 3.2Ghz chip to 3.4Ghz with a option in the BIOS for example). But for the lowend Dimensions it ain't something you can do. And it wouldn't be worth it either because it uses just a low end Celeron or P4.
if you whould have an non Dell mobo luke from msi, it whouldn't be a problem but as you are stuck with Dell i'm srr to say you won't be able to.
the intel 845 you have prob runs on a fsb of 333 like mine so not the 400 like a post above here.
and for the 1 that sais that it is only a P4 i have oc my p4 2.4Ghz northwood to 3.35Ghz, try doing that on your amd without dropping back to a fsb of 100 like amd likes to do now and then.