Sonic Firestorm is an excellent album for fast-paced melodic power metal, but it's worlds apart from Symphony X. I'd say SymX have some elements of power metal, but that's just one small aspect of the unique Symphony X style which encompasses traditional progressive rock, progressive metal, neo-classical metal, heavy metal, and many other genres.
If you liked Dragonforce, then here are some other power/melodic/speed metal bands of a similar sort of vein that may interest you:
Angra
Axenstar
Edguy
Firewind
Hammerfall
Kamelot
Rhapsody
Stormwarrior
Thunderstone
Wizard
Of these I would definitely recommend you check out Edguy, Firewind, Kamelot (last two releases especially) and Thunderstone. Oh and check out DF's debut - Valley Of The Damned. It's just as good as Sonic Firestorm.
If you're particularly impressed by guitar work, you'll probably also be awed by these:
Cacophony
Impelliterri
Jason Becker
Joe Stump
Michael Angelo
Michael Romeo
Racer X
Rusty Cooley
Yngwie Malmsteen
And as for progressive metal, check out:
Adagio
Ark
Dream Theater
Dreamscape
Threshold
Time Requiem
I cannot stress how good an album Ark's Burn The Sun is. It is brilliant. Oh and also buy all the Symphony X you don't already have.