I used Win98 at home until I broke down about a year ago and got XP. On Win98, I had been experiencing ever increasing problems, crashes, hangs - and one most irritating, no shutdowns, necessitating a power off, and the half hour scandisk - because I didn't shutdown properly, LOL. I kept updating frequently in hopes to rid the OS of these problems. XP has been working fine on the same HW. My Win98 machine at work does not have problems - company does not permit updates from MS, only through their distribution.
On Win98, I used a winmodem for years, and typically got 56kb connections using Win98. With XP I could not get anything over 28kb. A computer support person at work told me I'd have to go with a modem with an internal DSP to get the speed back up. I did, and managed to get speed up to 48kb. XP definitely is a performance drag over Win98.
No modem can get a connetcion faster than 53k over a standard phone line. Your actual connection is not necessarily what is reported by your dialer, because under some setups, the modem will report the actual connection speed, and under others it will report the rate between the modem and the computer. It depends on the brand and on the initialization string.
Windows tried to make the modem setup automatic, but unbranded internal modems often get get generic initializations that limit them to 28k. Beyond 28k, you have to send a brand specific setup to the modem to make it connect at at the highest possible speed.
Modems that work beyond 28k also have dynamic speeds. They adjust the transmission speed according to the amount of noise on the phone line.