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Not entirely sure of this myself, but here's what I know:
1) Sequential twin turbos - idea is, small turbo provides low down boost, then a larger turbo provides the boost higher up when the small one runs out of puff. About the only good thing to come out of this setup was the development of engines with turbo and superchargers (supercharger in place of the smaller turbo). Cars that use this setup include the Supra RZ, the Mazda RX-7 and the Subaru Legacy. Most performance modders remove the twin turbo setup, cos it sucks! The small turbos are often the same size, and (especially noted on Legacys) after only a few minor mods (exhaust, air filter, chip) - your small turbo cooks itself due to it running above it's safe speed.
OOPS - Edited cos I clicked the wrong button!
2) PROPER twin turbos - not sure the score here, but the two turbos work together - usually (I think
) one turbo feeds half the engine (like one bank on a V-8), and the other one feeds the other half. This way you get quick spool up AND good performance, as you can have a small turbo and it only has to feed 2 or 3 cylinders. Cars to use this type of setup include the Mitsubishi GTO/3000GT and the Nissan Skyline GTR.
Try searching on Google for more info on these setups - it's not something I spent much time looking at, as I more or less figure it was a bad idea for a 4 cylinder once I glanced at the subject.
For the most part you're right. Here's a bit of an edit job.
Sequential Turbos - This is a very effective street setup that is usually found on inline 6-cylinder motors like the ones found on the Supra (as mentioned) and the RB26DETT found on the Nissan Skyline GT-R. For huge power this setup is usually ditched in favor of one massive turbo.
As for replacing the smaller turbo which is used for low end by a supercharger, yes it has been done. However, this isn't considered an advancement from sequential turbos. It's not an easy thing to do since it's a plummer's nightmare and you usually need to find a way to control the switchover.
Twin Turbo - You're pretty much right on with this part. This is the setup found on V, W, and boxer motors. You were wrong in that this is the setup on the Skyline GT-R. It is the setup found on the 6G72TT 3000GT/VR-4 and MR, as well as the B5 Audi S4, and Porsche 911 Turbo.
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And even those people abandoned the idea as it was a waste of time and more importantly, their 
