Of course and of course. It’s all in the CSS.
To center (vertically) use margin:auto
#container{
width:750px;
margin:auto;
}
Unless you mean centered also horizontally. That can’t be easily done in CSS, but it is possible.
To have the drop-downs drop-up, just use negative values when positioning the sub menu ULs. Adjust as required.
Why are you using frames? That is a very simple layout.