Well, I am still saving up for many of the components for my upcoming rig, but when I saw the 1000W Antec PSU going for only $130, I HAD to scoop that deal up. If you have the $120 right now, I'd recommend getting this piece first, since the rest of the components will be following soon (by the end of December, as you said). Just my two cents.
edit: on a side note, I am going to have the cash for all the rest of my components by mid-late January, so I am looking to have my stuff ready even later than you. I just don't see me being able to save so much on that PSU as this if I get it all at once. Same would go for if I was considering the 1200. It's your decision though, and that is one of the great things about building your own PC; you make the calls, and you make it look like you want it