What do you need now?
If a GTX580 will do the job for you, now is probably a good time to buy. But do not plan on stepping up.
If, what you really want is likely a GTX680, which sounds to be a big jump over a GTX580, then it would be better to wait.
You will get what you want, and you will not be without a card during the step up exchange. The cost will be the same, since you will end up paying retail price for the GTX680 when all is done.
You might want to research the step up program faq on the evga web site.