Your first (and, basically *only*) question is whether or not the modem is compatible with Linux (most consumer modems are "WinModems", and a large number of WinModems *aren't* compatible with Linux).
In your case, the US Robotics usr5638 USB modem should work fine:
Second question: can your PPP/modem coexist peacefully with your DSL? The answer is "Sure!". Remember: they're two different interfaces. And Linux (like Windows, Mac and most contemporary OS's) support this. It's similar to adding another NIC (and possibly connecting it to a different subnet), to dialling in to a VPN, or even to firing up a virtual machine. No problem!
'Hope that helps .. PSM