Well is my OCZ ModXStream Pro 500w a very good powersupply ?
Is it the new or older version?
It will most likely run a 280x, but might depend on which version and if you plan on OC`ing it.
http://www.hardwaresecrets.com/article/973
http://www.anandtech.com/show/7406/the-sapphire-r9-280x-toxic-review/4
For a single 280x, I would be inclined to go for a decent 550W unit, at least then you have a bit of overhead.