This is what works for me...
I am not an expert at all and there are zillions of people better at making music than me... but I've made music for professional environments, for a few small time games that are currently in development, in games I've used for competitions during my extended stay at the university, and for a hobby (I'm an OverClocked Remixer) and let me be completely honest... my piano/keyboard knowledge is basic AT BEST, and I don't use a midi controller for any of my music endeavors. I exclusively use my PC and a mouse for music, and I just use a sequencer (FL Studio combined with good VSTs and Soundfonts), though I cater to a more electronic style. I'm not an expert by any means, but I just wanted to chime in and say that it certainly is do-able not to have a Midi controller to make decent music.
If you want a real sound get realistic samples. If you want chiptunes, then get an oscillator. Midi controllers are great, but you don't really need one. A knowledge of chords is more important, imo, which you'll develop naturally after messing around and copying your favorite song. You'll start seeing patterns and learning to hear the right thing. Studying in a formal setting never hurts either.
With the synthesizers and VSTs that are available now you can make music without any gear and just a computer and get a professional sound out of it. Live instruments are great, don't get me wrong, they have a real sound that you cannot reproduce, that said, if you're aiming for midi controller and want to make music that way, you could go with FL Studio and handful of soundfonts and VSTs.
But yeah, just wanted to chime in as a gaming sequencer. While you won't get a live instrument quality like you might with a midi controller for a select few instruments... like piano and guitar, which is important... but for strings ... might as well sequence them anyway. After a little studying and messing around with chords you can pin down most anything after a few weeks.... and then you can get an almost authentic sound from sequencing.
http://www.strike911.net/Music/STRIKE91 ... ce%20I.mp3
If you want to make an 8-bit sound or anything in between, you can get that heavily time based sound too...
That kind of stuff you would most likely have to tweak after inputting your midi data anyway...
Things that are difficult to reproduce though in my experience are acoustic guitars and electric guitars, though the VSTs they have out now, like GuitarRig, combined with something like FLStudio's Slayer plugin give you an almost real sounding electric guitar. Really cool stuff they have coming out now. AGain, nothing beats the real thing if you're going for that; the emotions from a real musician playing an instrument is just far more emotional... but for me, its been ideal to be a jack of all trades and I could only do that by knowing how to sequence (or I could become a one man band but I'm just not that talented).
Midi controllers are great for a lot of people, but not vital. What matters is whether or not you are able to make music more effectively (and efficiently) with a midi controller... which you'll know. IF you can get your note data into the computer easily one way then go for it.
So yeah, if you feel the Midi controller is more ideal for you then I say go for it, but in the end you're going to need software experience anyway to tweak the sound and use the right instruments anyway. And a knowledge of sequencing will have to come with that.
Also... if you ever need a generic musician, wink wink nudge nudge. lol.
(I apologize for the shameless plugs) but one more:
video game remixes, originals, and more...