The problem you are going to have is billowing with the vents right underneath of the drapes. The only wany to completely stop this is to cover the vents, or get one of those hideous plastic things that sit on top of it and blow the air across the floor instead of up. Even if you add weights to the bottom they are going to billow out. Otherwise, standard drapes usually hang anywhere from 1/4" to 1/2" off the floor for a formal look.

Unfortunately, kyocius is right with her assessment. For a more flush fall, you can try hanging them with a curtain track, however. I'll include the link to the site where I got mine.

Here is a link that might be useful: Curtain Track Web Site