Yes, the FPS may go up but slightly. It's not worth the extra money.
The Heaven Benchmark is GPU based application. Even all the good cards will perform a bit badly in the Heaven bnechmark. CPU bottlenecked only starts to appear when you're running maybe 2x HD5970s or 4x GTX480s or 470s.
You may see a boost but its not very much,also benchmarks like heaven aren't the best way to determine your system's performance,try some real-time games and check your FPS,if its low then upgrade your VGA.(However a 5850 can handle every game fine on 1080p)