I have a 4790K and a 770 Sli. I can say that one 770 is enough for most games at 1080p, while it was very beneficial to have two 770's in Sli for games like GTA V, where certain in game features can make frame-rates unbearable at times. 4770K is a very solid processor, and definitely not a bottleneck for you, you should keep that for as long as you can, get a couple years out of it at the very least.
As for the 770, you can upgrade this independently, and easily. And I would upgrade this when you start to notice performance hindrances. A solitary 770 will prevent you from playing some games (like GTA V) on max settings. However if you are a casual League player for example, this card won't hold you back for a long time. It really comes down to what kind of games you intend to play, and at what settings.
I would not worry about your parts degrading, the main factor is that some software will advance beyond your computer's capabilities eventually, but you have some time. Software will most likely convince you to upgrade, and not your current part's lack of performance.