R. Lee Erney definitely as the First Sergeant.
Sean Bean as Hammer or maybe Ratliff.
Sean Connery as Gunny Thatcher.
Jason Isaacs as General Billie (I dunno, Mr. Isaacs just seems to be such a good bad guy).
Tom Sizemore as Charlie Bass.
Eric Bana as Captain Coronado.
Adam Baldwin and Nathan Fillion would both be in it but more as bit characters or minor characters.
Sam Shepard as Brigadier Sturgeon.
And as Mr. Cragg wrote, a bunch of young, unknown actors for the young Marines (Dean, Claypoole, etc).
I definitely would ban Christian Bale, Ashton Kutcher, and Hayden Christensen from ever participating on this project. They majorly lacking in talent.
I'd try to find parts for Christopher Lee and some others that I can't think of right now.
Shadowghost1
Sure there are places for Christopher Walken, Tommy Lee Jones, and Harvey Keitel. I just can't think of what specific parts they'd play. :-P