Hey, I went through this exact same thing when I was considering islam, and then after I converted. I was in college, and dependent on them financially. And, I was a huge chicken, and didn't want any confrontation. So, I didn't tell them. Unfortunately, they suspected something was up, snooped in my email, and we ended up having a huge confrontation anyways.
Alhamdulilah (praise God) though, in the end, they've come to a quiet acceptance of it. They pretend it didn't happen, and we still get along
Here's an article aimed at converts telling their families -
http://web.archive.org/web/200708140...com/family.htm
but in the end, really you know your family best. If you really think they'd disown you, keep it quiet. Don't leave any books on islam laying around where they could find them, and always clear the cookies and passwords off the computer that you use when you're home. If/when you do convert, and you're financially stable, I'd suggest writing them a letter. Emphasize that you are not doing this because you hate them, and you still honor and respect them (Islam tells us to respect our families, etc), but that you needed to find the best way to know God. You still have a tremendous amount of love for Jesus, etc etc etc.
I would start preparing what you would say to them now, just in case they do find out, so that you're not caught like I was, completely unawares.