If you're overclocking, the 7950 is a much better buy. Using the same site:
http://www.directcanada.com/products/?sku=11830BD3894&vpn=GV-R795WF3-3GD&manufacture=GIGABYTE . The GTX 670 will be faster at stock, but once both cards are overclocked they'll be neck-and-neck. That said, the GTX 670 wouldn't be worth the extra $60. I think in either case getting the Gigabyte Windforce 3 is a great choice.
As far as upgrading goes, when you need more power just grab whatever newest GPU is out then. Multi-GPU is rarely a good idea and it's even rarer that it's a necessity.