Well, Honestly... You can probably salvage the CPU. What is it a P4 2.8Ghz? Not too bad... Would give you something decent for gaming.
And you could use the HDD & any optical drives.
But, You'd need, "A new motherboard Socket 478 or 775", You'd need some ram... ideally 1GIG. Then You'd need a new case, because the new motherboard won't fit in the Dell case. And the dell motherboard won't fit in the new case either... Then you'd need a PSU, if the new case doesn't come with one. Oh and you'd have to pick up a OEM Copy of an operating system like XP home or Pro. Unless of course you were going with linux. You can't use the Dell OEM OS on another PC.
SO yeah, you're better off building a whole new PC.