Is an ANTEC Smartpower 2.0 350 watt good enough for a 6800gs?
Take a look at this PSU wattage calculator. Just add a further 30% to whatever figure it gives you, for a safety margin.
I guess it also debends on the rest of your system, i currenty have a 6800gs and run that on a standard 350w power supply with no probs (not even sure what make it is).
Here is a good link though about psu.
Click here before choosing a psu.