Well, I saw your other post about temps in the 90's with X1900 - and to be honest I thought 90 wasn't too terribly surprising for these chips (which run hot). I mean, in a previous review of graphics coolers (focusing on the HR-03), from Tomshardware;
http://www.tomshardware.com/2006/11/14/thermalrights_the_hr-03_is_a_vga_cooler_gorilla/page4.html
....you can see that the stock cooler has the card sitting at 89C under load.
You can also see that 77C is pretty good for your AC X2. In the review, they get 81C with the AC X2 when the fan is 100%, and 88C when the fan is on 'auto'. They're using a X1900XTX rather than an XT mind, but I wouldn't have thought that would make too much difference?
EDIT: Actually, re-reading the other post, you said you were up at 100C, which IS starting to push it I guess - but I figure that's kind of irrelevant as you're sitting pretty at 77C now - which is fine in my book.
If you really must have a super cool (air) cooler, I can totally vouch for the HR-03, from the above link - don't know what sort of return policy you have on the X2. The HR-03 IS more expensive - but then an extra 30 bucks didn't bother me when I was spending 2000+ on the system.