hate to tell ya this 24k but no "tweaking", modem changes or even isp changes in most cases will help you, because its fair enough to assume the telco thats in your area uses the dreaded "pair-gain system" (im a victim of this too ) which will make you always connect at a max of 28.8k (like i am)
heres a definition of it:
pair-gain system: A transmission system that uses concentrators or multiplexers so that fewer wire pairs may be used than would otherwise be required to provide service to a given number of subscribers.