I think, that however you decide to come out, you should wear YOUR clothes. The clothes you feel good in. Because if you wear girl clothes, your mom's likely to... not believe you, or just think you're patronizing her or adding extra stress for no reason.

I don't know your mom, of course, so I couldn't even begin to speculate on how she'll react, but she could very well run the gamut of emotions. Denial, ignorance, anger, disappointment... Ultimately, though, you just have to trust that she just loves you and wants you to be happy-- and once she cools off and gets used to the idea, then comes the acceptance.

When I came out to my mom, she didn't believe me. Neither did my brother. My brother still seems to think that I'm going to suddenly turn 40 and realise I was wrong, and not be able to 'change back.' He wants me to be happy, but... he seems to think that it's more important that I'm happy when I'm 40, than now. Go figure.
My mom thought I was trying to emulate Izzard, since I was bonkers obsessed over him at the time. She basically thought I just got the idea in my head and went along with it, deluded myself.
For a long time after I came out, it was just business as usual-- we went on as if I'd never said anything. And then eventually, my mom pretty much came to terms with it. Now all she worries about is my health when I start T-- she's worried it might cause problems. She also says I shouldn't cut off my boobs because they might come in handy some day when I need to manipulate some guy (said tongue-in-cheek, of course, but she's serious about it too).

I know that probably doesn't help you too much. My coming out wasn't especially traumatic but it still hurts when someone you love just doesn't believe you about something you know to be true. But it got better.

Good luck with whatever you decide to do.