There is a fair amount of info on this on this thread.
http://forum.x-cart.com/viewtopic.php?t=677&highlight=
The file to amend is the the customer/products.php file
Line 61 is:
Code:
$search_query = "($sql_tbl[products].categoryid='$cat' or $sql_tbl[products].categoryid1='$cat' or $sql_tbl[products].categoryid2='$cat' or $sql_tbl[products].categoryid3='$cat') and $sql_tbl[products].forsale='Y' group by $sql_tbl[products].productid order by $sql_tbl[products].orderby ".($config["General"]["product_order_reversed"]=="Y"?"desc":"asc")." limit $first_page, ".$config["General"]["products_per_page"];
Change this to:
Code:
$search_query = "($sql_tbl[products].categoryid='$cat' or $sql_tbl[products].categoryid1='$cat' or $sql_tbl[products].categoryid2='$cat' or $sql_tbl[products].categoryid3='$cat') and $sql_tbl[products].forsale='Y' group by $sql_tbl[products].productid order by $sql_tbl[pricing].price ".($config["General"]["product_order_reversed"]=="Y"?"desc":"asc")." limit $first_page, ".$config["General"]["products_per_page"];
I have left this with the product order reversed bit in so that you can set to increase or decrease through the admin centre.
You will also need to amend the featured products php file in the same way if you want that to display in a similar way.
Enjoy.