The pentium III has models for slot 1 and socket 370, as well as mobile socket.
If you are using socket 370, the best you can upgrade to would be a pentium III 1.4GHz
However without your model number we don't know which socket you have or if the PIII 1.4GHz is on Dell's support list.
I would generally advise keeping old computers "as is" though because the performance gain/price (unless you find the part for free or for a low price from someone selling a used computer), is not worth it- assuming you are even able to find one.