I doubt that it's impossible, but it would require some modifications.
http://docs.us.dell.com/docs/systems/dalex/board.htm
The machine uses nonstandard power connectors. In addition to the
standard-looking 20 pin ATX power (which probably has nonstandard pinouts),
there's a proprietary 3.3V connector.
The front panel switches and indicators use a nonstandard connector.
The Dell case has six card slot openings (one ISA, one AGP, one shared
ISA/PCI, and three PCI). Seven would be more standard for that vintage,
although recent mainboards may have fewer (and no ISA slots at all).
It has a single serial port; two would be normal. You may have to modify the
case to take a third-party mainboard.
(My belief is that Dell does this to make it more trouble than it would be
worth to install a non-Dell mainboard in one of their systems. Profit
motives aside, imagine the service tech who is called to repair a Dell, but
finds that it's not a Dell inside any more.)
Also, P4 systems need ATX 12V power (normal ATX connector, plus a four-pin
one for additional current at 12V). (I don't know what an Athlon systems
need.) The Dell power supply is also pretty weak by present standards - it's
a 200W one. (Dell now uses a 250W supply in their P4 desktops, which would
still be considered small for anything but a compact system.)
In brief, re-using the case and power supply is more in the line of a stunt
than a practical upgrade. If you're using a recent mainboard (P4, P4-based
Celeron, AMD Athlon), you'll need new RAM (DDR rather than PC66), and you'd
probably want a newer (faster) hard drive.
Sorry that I can't be more encouraging.
Bob Knowlden
Spam dodger may be in use. Replace nkbob with bobkn.