Overclocking isn't for beginners. Read a lot about it if you decide to try it. I doubt you can do it with Original Equipment Manufacturer (Dell) anyway. They don't allow the option in their BIOS.
Overclocking causes a system to run less stable and generally only boosts performance by about ten to twenty percent. If you do it wrong you can damage your system components.
I would seriously think about buying another power supply to replace the poor quality one Dell will likely give you.
And even then, WILL you be able to replace the PSU? Dell uses a BTX as a standard for their computers. Last time I heard, $300 or so for a PSU that was 100W higher.