nVidia's site states 550w PSU minimum for the GTX275.
While a good PSU means you have a little leeway with it... I'd tend to err on the side of caution. If it doesn't work or you have stability problems... it'd probably be the PSU not being up to it.
@2air
there' no problem with a 500W PSU at all
as long as the PSU is a good one (which means it's backed by good reviews)
in this article : http://www.behardware.com/articles/670-1/pc-s-actual-power-consumption.html
you can see how consumption is often overrated (still as long as the PSU is a good one as said before)
even though the article is a little old with a little research you can update for more recent configurations : http://www.behardware.com/articles/755-4/report-radeon-hd-4890-vs-geforce-gtx-275.html
and bear in mind that these values are taken at the plug which means actual consumption is lower (if we target for a 80% efficiency that means that for example, with a whole config with GTX275, MAX actual consumption of 446 at the plug is more around 350W which then means that you got 150W left before the "limit")
edit: your antec 500W earthwatt is enough according to the reviews I read
it's 80+ and it seems there's no particular problem with it. you might want to know that it's actually made by "seasonic" which is known for the reliability of his power supplies. Not the best PSUs out there, but still quite good all around.
I say you'll be very fine as long as you don't overclock.