Form Close Or Dispose

I am using VB.NET (Visual Studio 2008) and thus .Net framework 3.5. I have developed an application which has a MDI form and then has several other forms, most of which are shown using form.showdialog and some set of forms are shown using form.show.Whenenver a form (shown using form.showdialog) was closed (usually by clicking on a button (OK or Cancel)) I wrote code Me.dispose. After careful testing and R&D I figured this caused flickering on the screen. There use to be a flicker on the screen and it looked weird.In some rare circumstances if I clicked on button and form closed using me.dispose the focus of application was lost and some other program that was running came on focus. (This has happened but was rare.)Now the reason I used disposed was that after a form was shown using .showdialog I didnt want its contents to be in memory so I used the .dispose thing. But I noticed that If I use form.close then there is no flicker and form closes fine. But I think that would not dispose the form from the memory.

What should I do so that form closes and is disposes as well without any flicker.?I read on net that for forms shown using .show it gets disposed off automatically the moment it is closed (by using me.close) or user clicking on cross. Is this true? I also think that If I call a form using showdialog then I can write formx.dispose from where I am calling and not in the form itself.