You need to re-evaluate your budget. It is senseless to try to upgrade an FX rig to 'hi end' gaming status. Sell the box whole,maxed out with the six core chip (clocked, cpu cooler,single gpu etc) and start fresh.
There are 1 or 2 games that work well with an 8 core (battlefield) but most dont.
Put the money towards an intel build.
Or even sell the rig and keep the graphics card.