I have experimented with guitar strumming by "playing" guitar via Guitarism iPad app and sending the midi out to various guitar samples. It still didn't sound as realistic as with Guitarism's own sounds.
I think what you need is the typical hard friction sound of a guitar string that is often missing in soft guitar samples.
If you try the Roland SN-A strumming guitar, this is exactly what you notice when playing a chord versus single melody note. SN-A not only converts the simultaneous chord into a quick arpeggio, but also changes the "samples" to a more friction-sound sample.
Hope you can use this observation...

I appreciated everyone's responses, but I didn't clarify. I was actually looking for how a guitarist puts his chords together...how he strums them and what notes. Dave said an A minor chord starts on an E. I wasn't aware of that. Hence the different sound.

Anyway I found this video that spells it all out for me. And how other instruments are simulated.

Well, that's only if you're strumming all six strings. You don't HAVE to. In fact, when I was just learning piano I started transcribing the notes from my guitar and I played my D chord with an E on the bottom! YIKES. What a noob I was. As I progress in my guitaring, I find myself using fewer notes. I'm really trying to make the notes count, instead of just filling up all the holes.