Getting the names of the fonts is pretty easy. I like Heike's solution, but I can remember the following procedure better.
Just format the text in some text cell with any font style you like and then enter the menu command Cell > Show Expression (ctrlshiftE on a PC) revealing the formatting instructions.
Cell[TextData[StyleBox["aaaaa",
FontFamily->"Myriad Pro Light",
FontWeight->"Demi",
FontSlant->"Italic"]], "Text",
CellChangeTimes->{{3.543584575138007*^9, 3.5435845788282185`*^9}}]
Using this information:
Column[{
Style["Myriad Pro", FontFamily -> "Myriad Pro", 80],
Style["Myriad Pro Cond", FontFamily -> "Myriad Pro Cond", 80],
Style["Myriad Pro Cond Italic", FontFamily -> "Myriad Pro Cond",
FontSlant -> "Italic", 80],
Style["Myriad Pro Cond Bold", FontFamily -> "Myriad Pro Cond",
FontSlant -> "Plain", FontWeight -> "Bold", 80],
Style["Myriad Pro Cond Bold Italic",
FontFamily -> "Myriad Pro Cond", FontSlant -> "Italic",
FontWeight -> "Bold", 80],
Style["Myriad Pro Cond Semibold", FontFamily -> "Myriad Pro Light",
FontWeight -> "Demi", 80],
Style["Myriad Pro Cond Semibold Italic", FontFamily -> "Myriad Pro Light",
FontWeight -> "Demi", FontSlant -> "Italic", 80]
}
]

By the way: Although it is often quite possible to specify slant and weight in the font name, there is a good reason to specify them separately. Compare the parenthesis in the following:
Style["Text (x) Text", FontFamily -> "Myriad Pro-Bold-Italic", 80]

and
Style["Text (x) Text", FontFamily -> "Myriad Pro", FontWeight -> Bold,
FontSlant -> Italic , 80]

For a lot of symbols, Mathematica substitutes the Mathematica font version for the one in the specified font. If you have specified slant and weight in the font name MMA doesn't pick up those font specifications, ending up with symbols that are unmatched in style to the rest of the text.
Cases[FE`Evaluate[FEPrivate`GetPopupList["MenuListFonts"]], PatternSequence[a_String -> b_] /; StringMatchQ[a, "Myriad Pro*"]:> a]– Heike Apr 16 '12 at 12:53Cond,SemiCond,SemiExtorExtfor the condensed, semi-condensed, semi-extended and extended fonts. This is borderline buggy, because the full name reported byotfinfois, e.g. “Myriad Pro Condensed” for the condensed variant… – F'x Apr 16 '12 at 13:05FontFamily(You can do that most of the time but there are drawbacks). Instead, you should useFontWeight. – Sjoerd C. de Vries Apr 16 '12 at 13:15