A 6850 is a pretty good card to pair with that CPU. A better videocard will only help in games that only need a basic dual core CPU and 4gb system ram. GTA V, to pick a popular example, needs a quad core and 8gb to run smoothly. It would help to know 4 things:
1. What games do you play/want to play?
2. What resolution do you want to run?
3. What do you consider 'smooth'?
4. What kind of power supply do you have and how much power is it rated for(ie 300W)?