Don't start off in a crossfire/sli setup. It limits your upgrade options in the future and can cause undue headaches in some titles that may not support dual card solutions 100%. My advice is always to buy the best single card you can. That way you can crossfire/sli in the future if you so choose. Now if you start off with 2x670/680 or 2x7950/7970, that's another story.
Personally I'd say grab a 670. A stock 670 runs within 5-10% of a 680 for $100 less and you can get a factory OC'ed 670 that will match/beat a stock 680 for a slight premium over the stock 670 price. IMHO 680's are not worth the cost increase for the slight performance bump, unless of course, you have the money to burn and want the absolute best. Otherwise the extra $100 is better spent on a SSD/CPU cooler or saved toward your next upgrade.