Best way to check is take the side of the case off and look for the specific motherboard installed on your PC. Dell often use different makes of board inside the same series of pc's. Once you have the Mobo maker / number you can check for sure on the manufacturers website. Otherwise you may end up with a CPU that isnt compatible.
Manufacturer: Intel (OEM for Dell)
Dell Part#: M3918 DH682 Dimension 4700
Chipset: Intel D915 D915GOK 915G Express
Front Side Bus Speed: 800Mhz FSB
Supported CPUs: Intel Pentium 4 processor 520, 530, 540, 550, 560, or 570 with HT Technology - Intel P4 that runs at 2.8, 3.0, 3.2, 3.4, 3.6, and 3.8 GHz
not sure if a CPU upgrade is going to be worth while - depends on what you use it for.