I would go safe and get an 800-900 watt PSU. If the PSU's your looking at are around 300 bucks then get the Ultra 100 Watt PSU, it's modular and a great buy with plenty of room for expandibility.
$300??? why on earth would someone pay that when Corsair makes an excellent 850w PSU
that can be had for 129.99.
I see Jared beat me to it...stupid URL button sat there "loading".
But u have SLI on an AMD motherboard??