My first 5-box lvl 70 team was 5 x belfs. 1 Pally and 4 priests. Works like a charm for instances without fear (this was before the Fearward was added for all priests) However it was a HUGE misstep for pvp purposes.

Take it from one that has made his third lvl 70 5-box team: If you want to be serious about pvp, go for either Shamans or some combination of undeads. Everything else will just get you way too fustrated because of fearbombs.

My actuall pvp expirience so far (I've been 5-boxing since TBC released):
1 Pally + 4 priests (all belfs) : Close to useless for serious pvp.
1 priest + 4 locks all undeads) : Very good at BG's and world pvp. Limited in Arenas. But it requires ALOT of expirience to play well. You will have a ton of keybindings, X-keys PRO or something alike is highly reccomended for this.
5 shamans (orcs): Easyest of the setups. Relativly simple to controll (do not take me wrong on this one, it is stil HARD to master, but it is simpler than the lock+priest setup) . Also the best both for world pvp,BG and arenas IMO.