Well, Epica definitely have a very neo-classical/progressive symphonic sound. If you're looking for other bands with female vocals, then:
Nightwish (obviously)
Edenbridge (very neo-classical/progressive)
Lunatica
Visions of Atlantis
Nemesea
Xandria
None of these bands have quite the same sound as Epica. Personally, I prefer Simone Simons when she's singing with Kamelot.