Help on 5970 550 watts

Your going to need a good quality 750w+ PSU to power your setup.
Also does your case have room for the card?

Since the card your purchasing costs a hefty $640-$800+~ a really good PSU is in order to ensure it can feed the power it needs and remain completely safe from blowing/taking the card with it.

Corsair 850w 80 plus Silver.. 70a on 12v rail. As well as being modular. The reason I recommend this over the 750w model from Corsair is because the 850w will help with the PSU aging, and give you more head room for upgrading components such as hard drives, optical drives, fans, other PCI cards. etc.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

A 550 watt power supply will not be sufficient for a pc system with an ATI Radeon HD 5970 video card.

Here are the official power requirements for the ATI Radeon™ HD 5970 video card.


ATI Radeon™ HD5970 System Requirements:

PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard

650 watt or greater power supply with one 75 watt, 6-pin and one 150 watt, 8-pin PCI Express® power connectors

850 watt or greater power supply with two 75 watt 6-pin, and two 150 watt, 8-pin, PCI Express connectors for ATI CrossFireX™ technology in dual mode

The power supply recommendations are for an entire pc system.

This is what I would do if you want to find out what your system PSU needs are:

http://extreme.outervision.com/psucalculatorlite.jsp