Get one 7970 now, and another down the road, that's my plan anyway. You really wont come across a game you can't max out with one 7970 outside of running eyefinity.
An overclocked 7970 is running really
close and surpassing the 6990/590 in some cases. You also don't have to deal with the issues of SLI/Crossfire.
There are very few situations where you'd need more gpu power than this, Short of a 5x1 or 120hz 3x1 or 3x1 2560x1600 setup.
For any single monitor setup it's going to be just fine.