If you jump to a P4 system, you will probably also need a new video card if the motherboard doesn't have integrated video, a new PSU, and maybe a case fan or two. If you need a new PSU, you might consider the bargain of a new case bundle that comes with a PSU and an exhaust fan or two. Of course, your budget will help you make this decision.
Really, the only thing keeping you from easily upgrading in your existing case is if Dell used nonstandard case, motherboard, and PSU. This can be solved, however, by a little modification with a Dremel-type power tool.
Give us some details about your current system:
What Dell model number?
You may benefit from looking atthis discussion:
http://reviews.cnet.com/5208-6142-0.html?forumID=68&threadID=134052&messageID=1510382

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