Would it be a good decision to wait till Sept to get a GFX CARD?

The next generation of 28nm cards are not due out untill Xmas, or perhaps even next year early.

If you need one now, get it. If you find you need something stronger later, just sell it, and get the next best thing.

If your target date is September, wait, you will then have a better idea of when actual launch dates are.
At that time, consider a EVGA card with their 90 day trade op polisy.