In short:

Great Kilt=Belted Plaid (2 names for same thing)
This is the one that is historically proven from 1596-1800.
See "Rob Roy" with Liam Neeson (sp?) and Jessica Lang and Tim Roth (among others) for a decent representation in an easy media form.



As for the "Ancient Kilt," I am assuming this is refering to the offering by Celtic Croft/Kilts-n-Stuff.
http://www.kilts-n-stuff.com/Kilts/ancient_kilts.html
Note the first line on the page:
These are the same style kilts worn in the movie Braveheart.
followed by:
Depending on who you talk to, they may be historically accurate or not . . .

It is an attempt to make a "pre-belted plaid" form, which is not really proven by any real documentation, though it has the public imagination and would not het much arguement at most Ren Faires (though it might at an SCA event).

These are based on the movie, which has many historical dress errors, as well as historical IMPOSIBILITIES and anacronisms. Despite this, it remains one of my favorite movies. My biggest "issue" is the end, which is a slight to Robert Bruce. I even wrote a new ending that gives Bruce his proper credit.

Now, back to topic . . .

The questions asked:
Newbe question alert! Could someone please tell me the difference between an Ancient Kilt and Great Kilt? Which one is historically accurate for the 18th century?

Part 1: I hope the above answers that one.
Part 2: The historically accurate one for the 18th C. is definately the Great Kilt (though I prefer the term "belted plaid" for the same thing)

Hope that helps.