For brevity's sake, I'll mash both of yours together to answer the questioned ones:
Vangelis - Composer for many movies and various other endeavors. Probably his most notable compositions: Chariots of Fire and Blade Runner
Ayumi Hamasaki - Japanese (~Bubblegum) Pop singer.. uhm, yeah, you can go from there if you want.
Delerium - They've been on the dance/electronic music scene for quite some time. A lot of their stuff is more on the trance/ambient side for myself.
Roxette - An 80's Swedish band. They've had a few notable releases, although they escape me at the moment.
Sarah Brightman - Opera singer, well known for certain renditions of various operatic songs. Phantom of the Opera was apparently written for her voice.
Apoptygma Berzerk - Another electronic band that's been around for quite some time. They are Norweigan.
KLF - Very famous for their rendition of the Doctor Who theme.. although my personal favorite of theirs is Last Train to Transcentral of which I have several vinyls.
Dune - a lesser known band. Notable releases are Rainbow to the Stars, Million Miles from Home, and a few others.
Do as Infinity - Japanese rock band. They are apparently catching ground in the states, too.
BT - Brian Transeau, well known in the dance scene, but not so much in the outer circles. He is an accomplished composer doing scores for Fast and the Furios, Monster, Stealth, Go, Driven and Suspicion.
Yasunori Mitsuda - Main composer for the Final Fantasy video game series.
Danny Elfman - Previously of Oingo Boingo fame, this now composer does a lot of dark films and many TV series. His scores include Batman, Batman Returns, Edward Scissorhands, Corpse Bride, Spider-man, Spider-man 2, Charlie and the Chocolate Factory, and many many many more.
Robbie Williams - British ~pop singer. Hit American audiences big with "Millennium" back in 1998
ATB - André Tanneberger, german DJ/artist. Notable releases include
Marrakech on No Silence,
I Don't Wanna Stop on Addicted to Music, and
9 pm (Till I Come) on Movin' Melodies.