I have a HD 5770 in a CM Elite 335 case and I added the 120mm intake fan. My room temp is around 30'c (sometimes higher sometimes slightly lower), I live in a hot climate. My idle temps for my 5770 @ stock settings are from 34-40'c and load temps are in the 50's.
When I OC my 5770's Core from 850 to 940 and memory from 1200 to 1350 and fan to 70% my idle temps are 39-42'c and load are 59-64'c.
Try adding that 120mm intake fan.