It's not all nonsense, but be careful when deciding to use 'Internet Accelerator', some are fakes (specially P2P accelerators end up being trojans) and others can mess up the settings more (specially if you don't know what your doing and it doesn't have any backup). If you do use them, be sure to download from a trusted well-known company and the program that work with your specific operating system and network configuration. Some work fine, but be careful when picking. You also want to backup all original settings.
Consider making performance tweaks to your system manually rather than through use of a utility. This is advance tweaking, but gives you total control. Requires a bit of maths calculatulations (and a lot of registry locations to hack). However you'll understand clearly where the modifications were made and where they can be undone.
When tweaking the internet performance you want to first check...
What version of Windows or OS are you using?
Your router / dialup / cable / satelite, being used?
Your current net speed download, upload and ping to a server, etc?
Then go to your ISP and get advance settings from them if possible?
(important as this takes away a lot of the guess work and trail and error)
An online website or way of benchmarking each performance tweak test before and after - multiple times against the same server/ping) is required to ensure the tweak improved rather than just making it worst. Remember to reboot between tweaks to ensure the computer as set them and test them out!
Performance tweaks for low-speed (dialup) Internet focus mostly on improving download speed.
Broadband speed tweaks originally focused on increasing the performance of general Web surfing, speed tweaks are now more commonly made to tune specific applications like P2P file sharing systems and games.
The basic idea is to optimize net packets sizes, etc, to match those of your net speed, distance, line quality and ISP provider. This prevents most packet loss (therefore they don't have to resend the same data as much - you get it quicker), resizing packets to fit down the line (what your connection and ISP can handle at a time) and streamlines the connect to become smoother. You might get a 10-20% gain, depending on your connection and what your using it for. If broadband is capped - you won't be able to legally go over that cap but you can at least make it smoother performance.
On top of that you can setup QOS (Quality of Service) on your Router, this will proiry applications/games and direct them to the computer using that app/game. This is best for networked (shared) boardband when online gaming and another user is surfing the web, checking emails, etc.
The best performance boost isn't actually software, but checking your line for noise/static which would break down the signal. Cleaning the phone line, location of router, cables used/length, ensuring it's not affected by interference (specially if using wireless). If there is line noise / interference this can be the difference between slow/dropping net performance to high speed (100%-200% gain) with a clean connection.
