use coolbits and change the setting to Performane (3D) and click detect optimal settings..
it it says example 280 core and 420 ram
put it to that or a little under
by the way my friend has a pny 5200 128meg and he has his at 300/500 stable so maybe those cards can be overclocked very well
i have a 5500 256meg and mines at 300/451