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I could start a whole story why you should not listen to either Within temptation or Evanescence, but I think I'll just let that be.
Let me comment on Nightwish first. Nightwish has been around for about 8 years now. They released quite a few albums since then and there are huge differences between them. The latest two albums, Century child and Once are somewhat more commercial albums. In my opinion, they are good, but nowhere getting close to what they managed to do a few years back. You really should listen to their older albums, esp. Oceanborn. If you find that this genre of music is to your taste, Oceanborn will something you want to take with you in your grave. Actually, one song of Oceanborn will be played on my funeral (the funeral itsself isn't planned yet btw :p).
Of course, there are a few more bands that you might like. Let me restrict myself to the metal genre, esp powermetal, goth metal and all the epic styles (as they are somewhat related). Bands you could check out and most probably will like are Lacuna coil, The gathering, Epica (they kind of suck, but seem popular at the same time), Edguy, Avantasia, Ayreon, After forever, Anathema, Ayreon, Star one, Therion, Luca Turilli, Rhapsody, Kamelot, Sonata arctica, Stratovarious, etc etc... It's not all like Evanescence, but it's quite typical that ppl that like Evanscence thend to like these bands as well.
Tell us if you need to know more. There are quite a few nice metalleirs on the board so you'll be just fine