Radeon 7970 is much better. Crossfire is putting multiple Radeon GPUs together, and SLI is the same thing, but for Nvidia cards.
So can I crossfire it later on or SLI it later on and just buy one card now?
The 7970 is much better than the 760. And yes, if you buy one 7970 now you can buy another later and use it in crossfire. If you buy a 760 now and another later you can use it in SLI. You cannot mix AMD and nVidia graphics cards.
Edit: also, if you think you'd like nVidia and SLI more than HD 7970 and Crossfire, look at the GTX 770 card, not the GTX 760. The GTX 770 and the HD 7970 have very similar performance. A 850W PSU can handle GTX 770 SLI too.