a 450W Earthwatts or Corsair would be sufficient as timop stated
In 1-2 months they are releasing the 67XX series, again the 67XX series. Thats the midrange series, the 6770 will be at best as good as the 5850. It has 1400 stream processors and the 5850 has 1440. Its somewhere around there.
You can speculate on the 6870 series but it should be 3-4 more months, and upgrading from a 2600 to a 5870 is likely all he needs.
@ OP if you want to try a different card tell us what you are trying to run so we can throw recommendations other than that run a 450W PSU and you should be fine. For arguements sake i ran SLI GTX 280's on a 650W.
a ATi 5870 uses 188W of power, and id bet your cpu is max of 140W.
Which is a max of 328W which clears a 450W.