Always buy the single best card that you can, if later that single card is not cutting it then go sli or buy a new card. I've ran 460, 480, 560ti, and 580's all in sli over the years and have not had many issues. Biggest one was when an sli bridge went out and took me a few hours to figure it out. Some games wont run with sli/cf, there are very few games that don't anymore, but you can always go into the NVidia setting and force it to do every other frame per card. Some people will claim micro stuttering with lower end cards, I for one have never had a problem with the cards I ran or just don't notice it.
By the time you will need to sli a 770 a newer card will most likely be out that will outperform a 770sli setup. A few other things you need to look at when doing sli, how far apart are the PCI-e x16 slots, how thick are the cards 2/3 slots, will they get enough air to cool them, they will heat up the case more then a single card, power requirements. all of this need to be looked at when considering an sli setup.