Well i’m a dummies, try easy solution to get clear with ajax. So many solution here.Opensource is power,but lack some documentations and examples. Good ajax framework here : EXTJS and jQuery.

Grid: Extjs is good choice,but too complex for beginer, you can choose jQuery..maybe it more easier..

Just my humble opinions..

Good Links :

Extjs : www.extjs.com
jQuery : www.jquery.com
jqGrid : www.trirand.com/blog/

Cheers..