Also the main factor with any processing isn't the 'speed' but the algorithms used..
There is a world of difference between any manufacturers 100/200/120/240Hz processing.
In the past, with CRT's, the phosphors where designed to decay at a specific rate, not too quickly or you see too much 'flicker', and not too slowly as you see 'smearing' of fast moving objects. 100Hz came about, as it allowed faster decaying phosphors to be used, to ensure good motion detail, but where refreshed twice as quickly to reduce flicker. The other effect was that generally the 100Hz was achieved in the digital domain, so 'processing' was also done at the same time, all in an attempt to clean up the image, interpolate between frames to try and improve motion artefacts in the source, and add any number of other filters.
As Mr LOL points out, LCD especially does not need refreshing, so 100Hz/120Hz is largely all about the artificial processing of the image, 100Hz is a nice speed, because it allows an nice easy single 'inter-frame' to be created to allow smoother transistions of fast moving objects in the slower incoming 50Hz source material. However, this is all highly dependant on manufacturer. Toshiba and Sony pretty much have always provided some of the cleanest processing around (in general), Panasonic and Pioneer have also been notably good.
Of course, what most of it is there for is for 'films' which are filmed at 24fps/Hz, which doesn't fit well with the standard 50/60Hz video systems of various countries. 24Hz does produce noticeable stutter from being a low frame rate, and also, if it's encoded in to a standard 50/60Hz video stream, you get this periodic judder because 24 doesn't divide into 50 or 60 cleanly.. However if you multiply 24 * 5, you get 120Hz, which is also double 60 Hz (another popular frequency), hence why 120Hz sets have become more prevalent. To reduce the periodic judder, things like 3:2 and 5:5 pulldown (essentially just repeating frames over and over again), but you are still left with the natural motion stutter from 24Hz being so low.. This is where the fancy motion interpolation comes in, and again different manufacturers use different techniques. To do it right, a lot of processing needs to occur, and making this unobtrusive, and 'clean' is where it's at, notably Toshiba and Sony always seem to be good at this, Panasonic/Pioneer are also notably good, others tend to over process and create more artefacts then it needs to.
Recently, they found with LCD sets, that inserting a black frame between others, and doing this at higher rates (240Hz) etc fools the eye into more naturally motion smoothing, ironically, this is essentially mimicking phosphor delay, and one reason judder was less apparant on CRT's..
Plasma 600Hz is simply a high rate dark frame interpolation effect, it's refreshed at a high rate, it's a more natural way to reduce motion stutter without processing, of course, you really do need processing to really smooth it out, which is certainly not done at 600hz (there is no point).
So, as Mr LOL says, you really want to demo stuff yourself, and don't assume one mans 120Hz is the same accross the board, or that 240/600 is better because it's a higher number.
Some of the most overdone processing comes from Samsung, especially on their lower end 100/120Hz stuff, lots of edge enhancing and 'pop' added, but it's highly artificial, and certainly not my cup of tea, their higher end stuff is better, but IMO not as advanced as Sony, and not as artefact free as Toshiba (I used to work for Tosh.)
