How to fetch data for a large Gird from a database?

I have a grid with 10000 rows and 20 columns.
I want to make sure that the viewer will not experience a large delay when is browsing the grid. The size of display allows the user to see only 15 rows at a time.
I am thinking to open create an empty array which can hold all the data for the gird.
When the user displays the grid for first time, I will fetch the data for the first 15 rows and as he moves the scroll bar down I will go a fetch more data from server.
How do I get an event telling me the position of the scroll bar?
or what are the row numbers that are displayed after moving the scroll bar?

Thanks
MS

Maybe you should look at the code example in the middle of this topic -

http://www.activewidgets.com/javascript.forum.13524.6/when-will-ajax-paging-be.html

But first I would suggest actually trying to load the full dataset in the virtual mode and see if it really takes too much time.

thanks.