joe king is right; if you plan to stay on dial-up & not go broadband any time soon, you might be better off upgrading the modem, particularly if you have a winmodem.Have the new one put in an empty PSI slot, or, if you have no empty ones, have another card moved & have the modem put in the other card's slot, & it put in the old modem's spot.
You might try a trip to the modem mfr.'s site 1st tho, to see if there are any updated drivers available for your present modem. Also, note that if you have a fax or answering machine or another phone or such using the line, this can slow things down too.
You can also try an experiment: making the ISP switch to connecting via a long-distance call & see if the new phone route results in better modem performance, indicating a phone line problem.