As a trans-person, I figured I'd offer a little bit of info for this thread. First, hormones can do wonders, provided they are administered early. If HRT is started pre-puberty can have mind-bogglingly good results. HRT has less of an effect on the body as a person ages. For example, if a 22 year old transitioned, the hormones would have a greater effect on their bodies than a 32 or 42 year old. Basically, hormones become exponentially less effective as a person ages. What's more, hormones don't change bone structure at all. If you have broad shoulders before you transition, you will have them after (provided you don't have any major growth spurts left in you, hence what I said earlier about transitioning early). MtF hormone replacement therapy WILL change your primary and secondary sexual characteristics, though. This typically includes, but is not limited to: sterility, decreased (or non-existent) sexual performance as a male, breast growth (usually a size smaller than females in the immediate family), redistribution of fat to a more female pattern, a thinning of body hair (though not a complete elimination, hence why many trans-females have their hair (typically facial) removed via electrolysis or similar treatment), and a halt to hair loss (i.e. male pattern baldness). There are other effects, but those are the ones that came to me first. HRT does not change the voice, but it can be made to sound passably female with training (there are at least a few DVDs on the subject).
If anyone here is considering transitioning...it's a long, hard road. Even after the "physical" transition is done, the journey has just begun. People will perceive and treat you in a completely different way as a female (and and in yet another (often, worse) way if they know you're trans, but that's a different topic). I am by no means an expert on the subject, but I can recommend two good websites for those who are interested:
http://www.tsroadmap.com/index.html
and
http://www.lauras-playground.com/ (especially the forums).
I think the most important message in all of this is that you don't have to go it alone. There are people out there who have had similar experiences, and are happy to help you, if you look. Also, I don't want to sound preachy, but I feel I would be remiss if I didn't say: please, please, PLEASE don't self-medicate with hormones. That's a good way to kill yourself. Honestly, find a way to see a doctor...your transition will be much smoother and much safer if you do.
Okay, I'll go back to lurking now...