2003 is approximately the time of transition from IDE to SATA. Some Dell Dimension mobos had both IDE and SATA controller, so you might check yours.
Otherwise... if the hard drive's data connector is a wide, 80-wire ribbon, it's IDE.
There might also be a PCI board to enable the use of the SATA connector, but its throughput would be limited a bit by the PCI's max of 100-133 MB/s. That's probably faster than your present IDE drive, but not quite as fast as some of today's fastest spinners running from onboard SATA controller.