well,, the Rosewill will do better on both cards, but i would not suggest either. rosewill is more a bugget company and collermaster doesn't make good PSU's.
corsair, PC power and cooling, Seasonic, Antec (earthwatts series) thermaltake (though power series) PSU's are best.
I am powering a similar system (1 GB 4870 and two hard drives) with a 550 watt Antec. So, yes, a good 500 - 650 watt PSU will do nicely. If you know that you are not going to upgrade much, go for the lower end of the range. If you think you may upgrade, go for the upper end of the range.