The excellent Map Of Metal has got its act together and added a Symphonic Metal section! The site displays the history of metal as a map. You start at the 1970s on the far left and move to the right for newer genres.
The new symphonic metal section is an offshoot of Melodic Power Metal and lists its influences as Gothic Metal and Death Metal. There are a great choice of representative bands you can listen to also, including Nightwish, WT, Epica, Kamelot, Therion and Haggard.