Apple may or may not upgrade the GPU of the iMac in the next revision (Which is happening tomorrow or tuesday) but they most likely will do something to the graphics by September, if not a tad later. If you already have an iMac, I would wait until the graphics get updated, if that is not this upcoming revision, it definitely will be the one after. If however you do not currently have an iMac, then the next revision coming out in a few days should be just fine.
-robbier