If you're serious about choor music, get a nice system with Windows 7 64-bit, and a firewire recording interface... Does anyone really use a soundcard anymore? My interface is my "soundcard". I got 8gbs of ram that i'd be able to use fully if I actually had 64-bit versions of my programs. :3
Neblix still saying the anti-keyboard bit.. If you have any experience playing piano, you'll get more control using a midi keyboard.. even if you don't, you'd probably learn the basics within a month or two.. There's no rule that says you can't combine playing the keyboard AND clicking with the mouse. You'll get natural velocities if you play a keyboard, and your modulation and bends may not be perfect but they'd at least be legitimately performed, that is to say, they'll sound more real. Adding a tool can't hurt, it can only help. If you stick with just a mouse, then you're limiting yourself.
It also depends on your DAW of choice, if you go with FL Studio then by all means, you could primarily use the mouse as that is a DAW aimed more towards beats and sequencing than live performance or recording. I would personally recommend Cubase 5. As for learning a program, anything's going to look intimidating until you figure out what's up in it. At least if you learn a DAW like Cubase 5, it has a general structure that is similar to many other DAWs and the knowledge would transfer.