Downgrading XPS 420 Questions

First, here are my specs: Intel(R) Core(TM)2 Quad CPU    Q6600  @ 2.40GHz (4 CPUs), ~2.4GHz, 6 gigs of RAM, ATI Radeon HD 3450, I use Vista Home Premium 64 bit and i have a few questions and here are them. I'm planning to downgrade to XP home premium service pack 2.

-Will XP be able to utiilize all of my 4 cores?

-If anyone knows, what equipment will not be able for use in XP such as my monitor webcam and sideview on my harddrive?

-How will my gaming performance be changed? Increase or Decrease do you guys think? Such in FPS games as in FEAR, those kind of games.

-Will my warranty be affected?

-Are there any hazards about downgrading that i should know about? Besides losing vista programs and such.