Upgrade for Legion

I am trying to figure out how to upgrade my computer for World of Warcraft's Legion and I keep running into different ideas on how to fix my issues. Lag is one of them, and my frames per second are horrific at times. 

Currently I am running an x51 R2. My processor is an i3-4150 3.50GHz. My Graphics driver is a GTX 745. I know I need to upgrade something, but I don't want to drop a bunch of money for all but nothing. Can someone help me, please?