There are a couple of BIOS editors
this one recommended by Wazzledoozle which can let you view the different BIOS memory timings to get a better match for your card.. let us know what you find out ...
A quote from W1zzard ... "the next version fixes the timings editor + i added individual data files for r300,350,360,420 and rv350,360,370,380,410
in reply to dominik's email: i'm pretty confident you cant damage up your card by playing with the timings .. not 100% sure .. thats why the disclaimer is there."
The other editor is Rabit 1.5 Bios Editor ....