For cheaper ones:
1) 百利 mall in TST
2) 天man 台道in TST
normally a few thousands (probably 3k up) for rental package. There are lots of shops in 天man 台道, dresses are much much nicer than in 金都, price just slightly higher.
For expensive ones (for purchase only 4-6 digits $):
1) Central wedding (in Landmark)
2) Dorian Ho (Lane Crawford, Happy Valley Dorian Ho shop)
3) Walter Ma in CWB
4) BCBG in TST Harbour City
5) Matthew Williamson and other nice brands in Harvey Nichols
6) Swank in Pacific Place (carry brands like Valentino)
7) Vera Wang in Lane Crawford
8) Seibu also hold some nice dresses at cheap prices when on sale (1k+ up, worth take a look)