Welcome! I have a Droid and do not use slingbox but another member of DF is a good friend and he has used slingbox on his Droid. I imagine that you will be abel to use the X.
No, Droid X does nto have a 720 screen, so it won't make a difference. however if u decide touse it on ur laptop and u're streaming a HD channel, then it will. otherwise X will not make a difference