with that cpu you are not really going to get any benefit from anything more than a gtx460 or amd 6850. either of those cards may be too much for your psu to handle also. If you can get your cpu to 3ghz, that would be a good start. what make/model psu do you have? 2gb ram....upgrade it to 4gb especially if your running windows vista or win7.
i have a core 2 duo e6600 and it is VERY easy to overclock. gets from 2.4 - 2.7ghz without increasng voltage and 3ghz with voltage increase and stock cooler.. your 6570 should be able to get to 3ghz without voltage increase to make it stable. just keep an eye on temps.