I have the same model for the past 3 weeks and simply love it. You will not miss the faster video card at all. I have not found one thing that I would say this unit is slow at.
As for a newer spec machine. If you take a look at the intel roadmap spec, they will be introducing a new Skylake but it looks like the inital wave will be for the low power laptop headed units and not ones that would make it into an iMac.
Now what will be interesting is if Apple does stake the larger Broadwell E chips that have 6 and 8 cores and make a powerhouse computer with that. Now given the layout of the iMac and the heat distribution, i doubt that chip will ever hit a Mac. The Mac Pro all use Xeon chips so not likely.
I think you will be good for a while. In a year or so, they will have the newer Skylake chips in the iMac, but chances are it's likely to be that marginal improvements like between the 2012 and 2013 iMac's where there was a whopping .2 ghz step up. There will be some other internal improvements that go along with it like the bump to DDR4 memory chips, Thunderbolt 3..
But from the vast majority of users, it will make no real difference. The Retina screen was the huge jump forwards.