From what i know, you should atleast cover yourself up to your elbow.
As for pants, you should atleast cover up to your kneecaps.
i THINK all four schools of thoughts agree on that.
All i will add to it is, Dress modestly, not only when praying Salaat, but at all times.
Doesn't nessercerily mean 'always dress up in a Kurta ( http://www.eastwesteducation.org/ima...rta_pyjama.jpg ) and a Topi ( http://www.islamicjewellery.co.uk/images/csolcream.gif ), (heck, i NEVER do..), just dress modestly.