as previously stated by other members, overclocking a OEM PC especially DEll is nearly impossible, i think some do allow it but those would be the high end PC's with overclocking features that are made possible by the motherboard.
in any case P4's dont overclock heavily/very well, the max you would probably get the P4 is 3.1 or 3.2ghz, and from 2.88ghz thats not much difference, then you have to consider getting an after market CPU heat sink and cooler as the standard one will not cool the CPU properly.
my suggestion, if you have some money to spare......say $100 get yourself a nice socket 478 motherboard (if you still can????) thats capable of overclocking, then get a aftermarket heat sink&fan, then put the CPU into the new motherboard and experiment with overclocking.
BUT be very careful and read up about the overclocking process, as a few incorrect calculations and SIZZZZZLE there goes the CPU/motherboard. fried.
but if you buy a decent enough motherboard and not push the CPU/motherboard beyond the bounds of "safe" overclocking you will be fine.
and by "safe" overclocking i mean realizing when to stop before your PC becomes a paperweight ;)