7950 xfire will be faster and give more beef for your monitor, but 290X is a single card. It'll probably cost more though...
Your call.
Thanks, tough one! Would a Corsair TX 750 psu be okay for the job? Also, I'm unsure of whether 2 x 7950 IceQ Boost would fit? I have a Z77 Asrock Extreme 4 motherboard.