Oh boy, finally a chance for a monster post

A topic that was once very dear to my heart, Outworld are one of my favourite bands, but they aren't exactly power metal..
Depends what sort of vocals your after, if you mean high as in soaring operatic and like symphonic power metal check out Rhapsody of Fire, pretty good place to start on that front is here
http://www.youtube.com/watch?v=Ye6YHQ8AZzUAvantasia is another good band to check out in that sense, the metal opera had a killer collection of guest metal vocalists working together.
If you prefer a more straight forward brand of power metal a band like Firewind has excellant guitar solos (Gus G

) with a very good vocalist try here:
www.youtube.com/watch?v=WmIjCbnIPsI Hammerfall springs fairly quickly to mind if you like Sonata Arctica, they are fairly well known though would imagine you've heard of them..
Slightly more melodic approach is from Kamelot, Roy Khan's vocals are smooth like crushed velvet, one of my favourite vocalists full stop, try here
http://www.youtube.com/watch?v=0Qdcl67H1Jw (Guest vocals from shagrath on that one)
For all out ridiculously good vocals can't really look past Daniel Heiman, the range and power of his voice worth checking out, this is a song from Lost Horizon, which he has now left to currently sing for Heed.
http://www.youtube.com/watch?v=oZNZArVwXAU A shorter song, check out the song the highlander for their very best.
If you like Outworld another Progressive/Power band to check out is Dragonland
www.youtube.com/watch?v=rORt7fCcn2s Symphony X is another one to keep in mind for Prog metal as well as Pagans Mind being a slightly different approach:
http://www.youtube.com/watch?v=nJg33lYMEPYLast of all the ultimate power metal band would have to be Blind Guardian, one of the most epic songs ever made would have to be and then there was silence:
www.youtube.com/watch?v=snwvpJ7DxyYIf you like anything here and would like any other similar bands recommended just ask and Ill see what else comes up from my collection.
Cheers!