If you have a more modern bios it's just fine to pop in the ram, you should notice quite an improvement if coming from a low ram system. If not your going to have to do some small configuration in /etc/lilo.conf to make sure that your system registers the true amount of system memory.
The modem is a totally different issue. Winmodems can be tough to setup due to driver problems. If you can try to make sure you don't get one and get a modem that is supported in linux(various websearches can help you here). Of course if it's recognized and on the same com port you won't notice a thing different when connecting.