The graphics card market is very competitive today. You mostly get what you pay for. As time goes on, you will get more for your money, therefore "future Proofing" is not realistic.
A single 4890 will run games very well, even at 1920 x 1200. More expensive cards or configurations gain you less and less for your incremental dollar. I suggest you try just one, and invest the cash for a future upgrade IF you need it.