I would say keep what you have for now, if you want performance then invest in a better cooler and case, Like a Coolermaster hyper 212+ with 2 fans in a push pull config, pair that up with a better cooled case like a Coolermaster HAF 912 or 932 if you can push the budget that far. Then OC your X4 920 as much is stabily possible.
If that fails then you could pick up a AMD Phenom II X4 955BE for not much money and OC the hell out of it.
IMO you'd be better off simply replacing your graphic card and keeping the rest (as long as you don't plan to OC). But do keep in mind you may also need to replace your PSU with the choice of graphic card you make.