[Zippo44]

Weber's is unfortunately a militaristic Neanderthal in the worldview of the hollywierd libs, so the sweeping epic story of Commodore Harrington is unlikely to get picked up. As to Turtledove's alternate history...I can hear the shrieks of the libs now at the very idea that anyone could possibly contemplate for half a nanosecond the inkling of the idea that the Confederacy could have won the Civil War.
Nope, you're more likely to see a movie made out of Mein Kampf than those two.
Clancy is always a possibility, but they'll trash the story to make it politically correct.
Flint's series now...I received the second one in the series as a gift and refuse to read it until I can find the first one.
I'm not familiar with Ringo's stuff, but will seek it out on your recommendation.

[Centurion2000]

Unfortunately it's very difficult to find some of these books in liberal bookstores (B&N) but Waldenbooks still carry them. Amazon has all of them so you can find them there.

Also Baen.com has a webpage where you can read 1632 for free.

Free Baen library books

1632 and 1633 are in that list. Actually I just checked and you can download the books there yourself.
Gust Front : first of the Posleen war books is listed there too. Another one you can find that you might like it by Michael Z Williamson called Freehold : Excellent reading.

David Weber's Apocalypse Troll might be another good one that could be adapted to a movie.