Will I need a special power supply for this purchase?

Hey all -

I'm purchasing a new XPS 9000 and want to make sure if I need a power supply before I make my purchase to alter my budget.

I am planning on purchasing the XPS 9000 with the following hardware specs:

• Processor: Intel® Core™ i7-940 processor(8MB L3 Cache, 2.93GHz)
• Memory: 9GB DDR3 SDRAM at 1066MHz - 6 DIMMs
• Graphics: nVidia GeForce GTX 260 1792MB
• Monitor: 24" Ultrasharp

and if it matters...

650GB hard drive

Will I need a special powersupply for this set up? If so, how much? I don't believe I saw any available for purchase through the purchase plan, I'd prefer to buy aftermarket anyhow.

Also, what would make purchasing a Dell better than an HP, because I found one with almost identical specs for a bit cheaper.. can anyone persuade me?