If you are using WiFi then you will be limited to the maximum connection speed that the WiFi card inside the PS3 can handle. The Playstation 3 is equipped with a 54g wireless card which, like the name might imply, supports a maximum of 54mb. If you have a 100mb connection the PS3 will only be able to use 54mb of this speed over WiFi.
If is not possible to upgrade the WiFi chip that is used in the PS3. The only alternative that will allow you to gain faster speeds is to use a wired connection. There is a port on the back of the PS3 that will connect directly to your router. This will allow you to connect to the internet and take advantage of the faster speeds that your connection might have.