my last rig was e6750 oc'ed to 3.2 with gtx 260 but this card can go 100% with e8400 pushed to 4ghz so actually you already bootleneck you current graphic card so 2 sugestions
1- overclock the cpu and the gtx 260 a litle
2- buy a new rig instead spending money to botleneck a second card to gain...