I doubt that the Diamond would be too troubled by the kind of overclock that would match the XFX XXX edition. The more airflow you have in your case the better tho. The fan on the Diamond actually looks more like the revision 2/'egg' cooler that has be shown to be superior at cooling the card compared to the original cooler as seen on the picture of the XFX HD-577A-ZNDC. Altho the revision 2 cooler doesn't push air out of the back of the case, which is why good airflow is preferred when you have a card with that cooler.
Diamond earned a spot on my personal "Do not buy" list for a host of sins including (but not limited to) buggy drivers ("standard" ATi ones wouldn't work), buggy/useless web site, and utterly incompetent tech support.