from the rest of your spec, your current system is on the last mile of upgrade path.
IMO, don't spend too much on your current setup. Just get the cheapest card and save for a new build. Better performance per dollar that way. Once you save enough, you are in time for the yet to release ivy-bridge (6-9 months), as well as the HD 7000 series and the geforce 700 series (rumored that 600 series are named for oem card).