So I have to go to my boyfriends christmas party (yea I know that it is early to think about it but id like to get it over with). I figured that I would just wear my mothers tahari dress (thrift shop, $38). It is a long black dress to the ground, sleeveless with a low v neck and 5 or 6 jewels down from the point of the v. Is this too dressy for a semiformal party? Would it be OK and look less formal if I simply did no jewelry and a simple hairdo (just down or a low ponytail)?

It seems to me that the difference between semi-formal and formal means shortened hem length too. The dress you described sounds more like formal. You got it right in that both are dressy but formal more so. Is there a means to make the dress shorter, with a belt or actually hemmed?

So I have to go to my boyfriends christmas party (yea I know that it is early to think about it but id like to get it over with). I figured that I would just wear my mothers tahari dress (thrift shop, $38). It is a long black dress to the ground, sleeveless with a low v neck and 5 or 6 jewels down from the point of the v. Is this too dressy for a semiformal party? Would it be ok and look less formal if I simply did no jewelry and a simple hairdo (just down or a low ponytail)?
A floor-length dress or gown sounds pretty formal to me, but heck, it depends on what the rest of the party-goers are like. I'd try and find out what others are wearing. Me, I'd wear a 70's velvet gown from my thrift store collection to the grocery store if I was in the mood. If you love your dress and don't give a hoot what others think, then do what you got to do.