Speed booster wireless is just an enhancement, its not its own technology really.
Their is no real difference between any of them, they are still running on the 802.11a/b/g format.
SRX on the other hand actually does something, it claims that it can extend the range of your wireless network, it can, but not by are GREAT amount, you will notice atleast a 25% signal strength increase in those tough to get signal zones from this technology.
Speedbooster (linksys) SuperG Mode (dlink) and SRX, or any other technology that you can think of cannot increase your actual internet speed, they only increase the speed of your LA.
Hope that was helpful, there is probably something that I forgot to add, so others will fill in my gaps I suppose.