For a personal desktop machine you would typically choose the workstation install, which will install the components most commonly used in that type of environment. You can always add anything else later. I prefer to do custom installs, because you can pick exactly what you want to install from a list of all
available components. The downside of that is that is a) you have to wade through hundreds choices, and b) you can run into dependency problems, where one program relies on other software, but you chose not to install that other software. Theoretically, Redhat should warn you, and resolve the dependencies, but the process isn't perfect, to say the least.
A bit of warning: you might
run into problems with the workstation install's partitioning/formatting section if all you have on the drive currently are Windows partitions. I haven't had this happen myself, but I've read a number of posts by users who have.
As for the modem, I don't know. The install will try to set up your networking environment, but I don't know about the modem specifically; I use cable