http://usmilitary.about.com/od/theorderlyroom/a/citizenship.htmYes, non citizens can join and also obtain citizenship via that route if they choose to.
My brother was a Marine for 16 years and served in Iraq and Afghanistan with several guys in his unit who were not US citizens, most were hispanic but a few were from other countries, he said they were all great soldiers and he regularly put his life in their hands

I know the pay for a Marine starts off around $37,000 my brother had been in for 16 years and retired last august, he was making a little more than $60,000 when he retired and now gets a pension though not as big as it would have been if he stayed the 20 years.