You probably want to update your bios, and grab a quad core. Q6600s come cheap. I say upgrade CPU first, because of how old your dual core is, even if you had a Titan X not much would run on that system. I also recommend more RAM but thats not as urgent.
After upgrading to a quad core, maybe go for a GT 1030. They have slightly more power than an older HD 6870 from AMD, and they run cooler and quieter and are way more power efficient.