You will probably need a better PSU.
ATI Officially Recommends a 650W+ PSU for a single 5970.
I would recommend looking into a new Corsair, PCP&C, Seasonic, Enermax or Antec 650W+ PSU before upgrading to a 5970.
As with the above, any current single GPU card should work fine with your PSU.
Really, you should probably wait another week or two before upgrading your GPU.
Nvidia's Fermi is scheduled for a paper launch the end of this week.
Depending on its performance, ATI will probably drop their prices in order to undercut what nVidia can offer.