If you hit CTRL-SPACE you'll get the dropdown of available completions for what you've just typed.
http://www.sublimetext.com/docs/2/tab_completion.html
If you want the autocomplete dropdown to appear as you type then add this line to your User Preferences.sublime-settings file:
{
"auto_complete_selector": "source, text"
}
That should do what you're looking for :-)
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