generally speaking, number up to 100 should be written out as words; all others as numerals. (exceptions to be made for technical manuals, etc.)

as for the conjunction ("and"), the rule is the same for ALL numbers: NEVER use "and" (though nearly everyone does in common speech.)

except when writing a check/bank draft, i have never in my life had occasion to write (or see written) such large numbers in "word" form. (so, if anyone wants to write me a check for 1,520,725 dollars/euros/pounds sterling, what have you, i shall graciously accept it.)

possible exception (sorry... just thought of this one): if a very large number is "round" or exact, it is common to write it as words, or even as a combination of words... 4 million dollars, or four million dollars (the first, it seems to me, is more common.) for "vague" large numbers (ex. "billions and billions" ... i quote carl sagan), of course the words are used since there is no numeric form for these expressions.