(And the tests referenced by Brandon should not scare you from doing that). For fireworks just get a tripod, keep it at ISO 200 and you can successfully use your 18-70 lens. The tripod and a remote release is the key to fireworks.
Certainly not attempting to start a flame war, just a friendly debate, but why would you consider shooting 1600 on that body, especially for night shots where it's clearly going to show? I would say the high ISO performance of the a100 is on par with my D100 and I don't ever go over 400, there is simply to much detail and sharpness lost. Yes, you can try to get rid of some of the noise, but the detail is still gone forever. 1600 on that camera is quite terrible.
Brandon - Sony does have 2.8 zooms. And of course the Minolta AF 2.8's are options as well. We need to get you a bit more educated in the Sony system.
I'm always open to learning and I'll be the first to admit that while I try to follow every manufactures body releases, I don't follow the glass much, except in Nikon and Canon, and Zeiss, Voigtlander, Sig, Tamron and Tokina, specifically in F and Canon mounts. I gave a quick look on B&H's site and didn't see anything in a Sony Alpha mount that was a 2.8 zoom.
I'd tend to say the A580 is up there with the D7000 and K5 (both of which with it shares a sensor), and the 60D, as far as high ISO performance.
I'll throw this in the mix, it's well known that Sony makes the sensor in the D7k, though it's believed that it's based on a Nikon design. Nikon lacks the fabs to manufacture CMOS, LCD, etc and it's certainly cheaper for them to outsource fab work to the big guys in the fab industry.
This article;
http://www.cameralabs.com/reviews/Sony_Alpha_DSLR_A580/noise_JPEG.shtml pretty wells shows the difference in noise between a D7k and a a580. I disagree with the authors reasoning as to why the noise is better on the D7k, (and that they share the exact sensor) but everyone is entitled to their opinions.
Starting at ISO 400, you can start to see a mild difference between the two, especially in the shadows of the flowers in the right corner of the wall. Moving to the 1600 shot, on the left side of the wall you can see a good bit more noise on the a580. At 3200 the noise is pronounced throughout most of the frame, especially on the walls on the Sony. At 6400 and above, it's completely obvious. Specifically at 12.8k, the entire a580 image is soft where there is still detail on the Nikon side (look at the flowers). Even at 25.6k, to me the Nikon image looks sharper and retains more detail than the 12.9k Sony image, of course color noise has really taken it's toll on the frame, especially in the shadow area in the right corner.
Please don't think I'm saying that the Sony is junk. It's not, not by far. There is a $400 difference between those bodies and at that small of a price tag, $400 is quite a lot of difference. You're getting quite a bit more for the $400 than just image quality on the Nikon side, things like mag alloy bodies, weather sealing, etc. Some people want these things, some don't. There is no one perfect camera or even camera system (Canon has a few pieces of glass that I would love to have on the Nikon side!).
And who knows, maybe they are the same sensor and the Nikon processing is just a little better than the Sony (they are all jpeg shots afterall). The only really 100% apples to apples comparison would be on RAW images, of which I haven't seen any comparisons unfortunately.
Everyone's mileage may vary, but personally I find all these cameras plenty usable to ISO6400. As for F2.8 zooms, there are dozens available in Sony mount, ranging from 11mm to 300mm on either end, from Sony, Minolta, Sigma, Tokina, and Tamron. Sony's excellent F2.8 G glass and Zeiss glass is well known, from the wider end (16-80 Zeiss) to longer end (80-200 G). Sony's mount is surprisingly healthy, with over 100 current Alpha mount lenses and nearly 400 heading to the used market. Not as big as Canon or Nikon's collection, but not too hard to find what you need. It was a good point by the way to suggest considering a body upgrade, as it would make a huge difference, if the OP was willing to spend.
Is Tokina making anything (new) for Sony? According to their site their entire current lineup is only available in F mount or Canon. Again, not knocking Sony, I'm only going by info I come across while researching other things. I did some more research and came across a few 2.8 (straight, not variable 2.8-4, which the A-mount seems to have a ton of). Sigma appears to have a ton of glass available on the A-mount, which I didn't know. Like I said, I just went by what B&H currently carries, or at least what their search engine says they carry. I'm jealous of some of your Zeiss stuff. While we have some Zeiss on the F mount side, it's all manual focus. If I wanted to MF all day, I'd go drag out my Yashica 124G. MF is about worthless with little ones about.
It all comes down to what you need. Sony makes a nice DSLR (though I really do dislike their NEX system), as does Canon, Pentax, Nikon, etc. I just continue to stick with Nikon because of their reliability, selection of glass (especially in the used market) and other semi-pro features. I know I (often) come off as "OMG NIKON OR DIE!", unfortunately I'm a great speaker, not so much as a great writer
I don't hesitate for a moment to use ISO 1600 or higher if that's what I need to get the picture. ISO 1600 with a little noise but stopped motion is always going to be a far better picture than 400 or 800 and having blur or not getting the picture at all. It's about learning to use your camera to get the best results you can which is what the OP is asking for advice on.
Thank-you for your imput, passionate opinions and tech advice. It's alot to take in for this mind
). That's because the good night shots use a longer exposure. So do fireworks shots. 
