^Noticeable performance boost in what? Games? None. There are very few games still which utilise all the 4 cores on the die. Perhaps you may get a boost of about a couple of frames more but that's about it. A 5850 with your current cinfig IMO is the best upgrade that there is. Should see you breeze through most games without hiccups, provided your CPU runs at a clock speed of more than 3.8GHz to eliminate the bottlenecks.
Give a quick browsing through this article - How many CPU cores do games need? | bit-tech.net
Should help you out.
Give a quick browsing through this article - How many CPU cores do games need? | bit-tech.net
Should help you out.