The tempest is a ridiculously big "mid" tower. It has more than enough room for almost any configuration, and most are very happy with it. Personally, I am not a big fan of the 900's looks, and I've heard it gathers dust pretty quickly. That 1200 is on sell for only $120, and is better than almost any other case as far as bang for the buck today. I still am going with the NZXT 921 for looks and size, as well as low price today. If I needed slightly bigger, I'd definitely have gone with the 1200 though. Personally, if you were planning to spend $110 on that case, then $120 for the 1200 is definitely worth the $10 extra, as it is easily one of the most popular full towers. I'd pick that baby up ASAP, since the sale ends at 3am.
The Antec 300, 900, 1200 is an excellent case series. I have the 900 myself and have nothing but good things to say about it. I have never used the NZXT Tempest myself, but have heard good things about it.
Now thats a good price for the 1200, and if your worried about saving $10 on the upgrade to the 1200 from the 900, dont worry just buy it. I also like the mention of the CM 690 another good case, but after buying the additional fans you run up to the price of the 1200, so why not just buy the 1200 which contains the nice tri-cool fans. All it's missing is a single 120mm side fan, and for that I recomend just poping in a cheap low noise/cfm fan that and you be set.
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
Haha, I'm not even planning on building my comp until may/june but I got the 1200 today, I had to. And a case is something that will last, unlike a video card/cpu/even a power supply...If you know what I mean, 6 months away
Definitely get the 1200 on sale now if you have the $$. Parts like cases and PSUs don't go obsolete in a few months time, so it's ok to buy them early and fill in the rest of the parts later. Parts like CPUs and video cards it's always better to wait as long as possible, because the longer you wait, the more prices will drop. (with the exception of ridiculous deals on Black Friday, and things like that)