I definately recommend joining a PvP or an RP PvP server. My best characters are on a PvE server, and not a day goes by that I wish I could get them onto a PvP server.
As far as Race, it doesn't really matter that much in the end.
Class is what matters and it depends on what you want to do. If you like being up close and personal in both PvP and PvE aspects of the game, roll a warrior or rogue.
If you want to go max damage from afar, roll a mage/warlock.
If you want decent damage and like the idea of having a pet, roll hunter.
All of the other classes (Paladin, Priest, Druid, Shaman) end up healing in PvE, so it can be frustrating if you want to get gear to help out in the dmg category.
However, Paladin's, Shaman, and Priests can be very powerful if specced for PvP.
Druids are the odd bunch. They are extremely sought after because very few people play them. But this is because they are a weak in PvP, especially if you have to be specced for PvE. However, I've seen some pretty nasty Druid PvPers.
I've played every class, and I came back to the rogue, because its your job to be sneaky and ruthless.