customising div’s used in dynamic_widgets

I’m customizing a theme and I’m not too hot on php. I’d like to add a number to the widget class for each of the dynamic_widget elements.

ATM its printing the following html

<li id=”archives” class=”widget widget_archive”><h2 class=”widgettitle”>Archives</h2>

I’d like to add a new class called widget1, widget2 so it prints like so:

<div id="sidebar">

      <li id="archives" class="widget widget1 widget_archive"><h2 class="widgettitle">Archives</h2> 

    * .....
    * .....
    * .....

<li id="archives" class="widget widget2 widget_links"><h2 class="widgettitle">links</h2>

    * .....
    * .....
    * .....

I’m sure this possible but not entirely how to build it. Can someone help??

many thanks in advance……

D