I currently have the exact same computer as you, and I purchased a GTX 750 TI OC 2GB for it, people said it wasn't going to work because of the power adapter only having 100-240 watts, well guess what, It's been a year since I have my graphics card in my computer, I even upgraded the RAM, and it works like a charm. Actually I bought a GTX 1050 Ti 2 days ago
If someone doesn't believe me I can send proof but whatever. I had to create an account to say that YES you can upgrade your Asus K31ADE with a new graphics card/ram doesn't matter, you can. I also have 2 monitors plugged in, everything is working perfectly.