About the 780ti you bought of your friend, I am not exactly sure. It should be working well with that configuration.
I would advise on upgrading to 16GB RAM, as in the current age of technology, games eat up a lot of RAM...
Getting 16 GB will help a lot and besides, many gamers nowadays upgrade to 16 GB anyway.
But if you want to stick with 8 GB RAM< then fine, no problem mate. I think it will work