1000 watts is more than enough for a Gtx690, its even a bit overkill. However if you really want to upgrade in the future and cannot afford the Gtx690, then you can simply add another Gtx760 later on. It should perform similarly and cost a bit less. Asus also now makes a dual Gtx760 card, although it will likely be priced higher than two Gtx760's and perform similarly.
Overall as long as your power supply has enough power and your motherboard has two PCIE slots then you can later upgrade to running two cards.