You won't necessarily have to buy a new power supply. It depends on the form factor of your current PSU and also as obsolete99 pointed out, the length of your cables coming from your PSU. Chances are if you have an ATX form factor PSU you should be ok with the length of your cables. Although even if you don't you do have the option of using extension cables. Hope this clarifies a bit.
Hmm... I'm going to have to look inside my PC later today. Also i forgot to mention the computer it self is pre-built by dell.