you get a 1 year full warranty on any problems that arise, and a 3 year extended warranty for RRoD.
to minimize chances of problems make sure your 360 has plenty of breathing room... it doesnt matter if you stand it up or lay it flat as long as you make sure all the air vents are unimpeded.
edit: if you get RRoD your xbox wont boot to the dashboard and you wont be able to play games... if you cool your xbox down and it works again, go ahead and play it. nothing will void your warranty. mine RRoD'd and i let it cool down, played it for another 2 days before it RRoD'd for good. thats when i sent it in and got it back in 2 and a half weeks with no problems at all.
and like i said before if you get a cooler, make sure its the base one...
like this one