I have noticed a few caveats with ATI cards and CS:S having recently switched over from an 8800GTS. The biggest gripe is on de_westwood - on an ATI card, there is PERVASIVE mist/dust everywhere. It's so bad that I can't see anyone on the other side of the map from CT spawn. Switching to DX level 8.1 helps the issue a bit, but it's still present. I think everything is a little bit darker in general, but it's not too bad. I have also noticed that blindness/screen flashes in Warcraft:source are much more debilitating. This probably means regular flashbangs are more debilitating as well, but I haven't really noticed anything with FBs. If I had a single gripe it would be the de_westwood gimp, which is not such a bad tradeoff for better framerates.