HD6990 is sh*t if to compare with HD5970. First of all it's 'two way' heat dispersion makes more problems than benefit (if to use casing). At second it takes much more electricity on same mhash/sec (~20%) if comparing to it's older 'sister' 
Third - on same clocks HD5970 gives more mhash/sec so it's older tech makes more on your needs.
Anyway, on full load with one HD6990 you will have to leave it's fan at ~60-70% so forget your silent pc. These card fans (standart stock one) works like plane turbine.
Third - on same clocks HD5970 gives more mhash/sec so it's older tech makes more on your needs.Anyway, on full load with one HD6990 you will have to leave it's fan at ~60-70% so forget your silent pc. These card fans (standart stock one) works like plane turbine.