from pure math, 3.4/2.8=1.21. So at best 21% faster cpu.
If this is a complete linear application then your 50fps then turns into 60fps.
which is your best case scenario that you can hope for if you are completely CPU bound.
Realistically though, if you are actually GPU bound then you get 0% increase as shown in this article.http://www.techspot.com/review/458-battlefield-3-performance/page7.html
Why don't you try it and find out.
editted: forgot to add article link