I have edited my product listing quite a bit but I used divs instead of table cells. Even so, you can provide a set width for the table to limit the display to 5 columns. Just make sure that the product image and any other info you want to display for the image is put into a single <td>. You can experiment with placing your table tags in different places after the <!--Begin item_rows --> comment tags.
It's easier to strip out all of the original table tags and start from scratch.
If you're interested in a simpler way to do that, check out this thread:
http://www.viart.com/howto_simple_way_to_strip_out_table_tags_in_template_files.html
This is part of my product_list code:
<!-- BEGIN items_rows -->
<div id="style-wrap">
<!-- BEGIN items_cols -->
<a name="p{form_id}"></a>
<!-- BEGIN item_added --><div class="message" align="center">{added_message}</div><!-- END item_added -->
<!-- BEGIN sc_errors --><div class="error" align="center">{errors_list}</div><!-- END sc_errors -->
<form name="form_{form_id}" method="post" action="{products_href}#p{form_id}">
{hidden_parameters}
<input type="hidden" name="type" value="list">
<input type="hidden" name="rp" value="{rp}">
<input type="hidden" name="rnd" value="{rnd}">
<input type="hidden" name="cart" value="ADD">
<input type="hidden" name="add_id" value="{item_id}">
<input type="hidden" name="saved_type_id" value="">
<input type="hidden" name="tax_percent" value="{tax_percent}">
<input type="hidden" name="product_params" value="{product_params}">
<div id="style">
<!-- begin found_in_category -->
<div class="smallText">{FOUND_IN_MSG}: <!-- BEGIN found_categories --><a class="smallText" href="{products_href}?category_id={ic_id}">{item_category}</a>{ic_separator}<!-- END found_categories --></div>
<!-- end found_in_category -->
<!-- begin small_image -->
<a href="{product_details_url}"><img class="productImg" name="image_{form_id}" src="{src}" {width} {height} border="0" vspace="10" hspace="10" alt="{alt}"></a><!-- end small_image -->
</div>
<!-- begin manufacturer_code_block -->
<!-- begin item_code_block -->
<!--begin product_new_image--> <!--end product_new_image-->
<p class="item-code">Item no. {item_code}</p>
<!-- end item_code_block -->
<p class="listprice">As low as <strong class="aslow-price"> ${manufacturer_code} </strong>ea.</p>
<!-- end manufacturer_code_block -->
<!-- begin description -->
<!-- end description -->
