there are tons and tons of youtube videos on how to reflow the GPU and CPU. just need a heat gun that can reach 500 degrees some tin foil and the proper tools to take the board completely out of the casing. I did two today as a matter of fact one of them was one of the very first 360s ever released lol. both are working great and out of the other 4 I have 2 of which were also re flowed by a heat gun from harbor freight and they have been working over a year with 0 issues. if you cant find the videos or do but arent sure which is best let me know Ill find you the good ones.
Ohh and towel tricks have been known to harm other components too so dont do it, also some of the videos arent the best but two key things are this, dont ever EVER! move tilt flip or heat on an uneven surface when attempting the reflow, and two, be sure to protect all the capacitors and other plastic parts from heat by wrapping in tin foil
Edit: one of the best videos I have found
http://www.youtube.com/watch?v=1lRdAFyvrX4