Fastwrites is a feature of the AGP bus (it's called AGP Fastwrites). There is NO such feature on your PCI-Express card.
I assume you're having some problems with crashes in games and someone recommended you disable Fastwrites? This is normally a good suggestion. Fastwrites add little noticeable perrformance but can cause system instability.
However, as I pointed out, this feature is not implemented on PCI-E bus.
FOR THE BENEFIT OF AGP BUS USERS WHO MAY WANT TO KNOW HOW TO DISABLE FASTWRITES:
Assuming you have the ATI Catalyst drivers installed - go to the SMARTGART tab in Advanced Display properties (Display Properties -> Settings Tab -> Advanced Button -> SMARTGART). There is a setting to disable FastWrites there.
There may also be a setting in the BIOS menu to disable AGP Fastwrites. This setting would override the one in the SMARTGART tab (the setting in SMARTGART will be grayed out if it's disabled in BIOS).