the product list displays in a table so dont think you can squeeze an <hr /> in there
but you can try adding a bottom border that would give you that separator look
find in your stylesheet (line 1095)
.centeredContent, th, #cartEmptyText, #cartBoxGVButton, #cartBoxEmpty, #cartBoxVoucherBalance, #navCatTabsWrapper, #navEZPageNextPrev, #bannerOne, #bannerTwo, #bannerThree, #bannerFour, #bannerFive, #bannerSix, #siteinfoLegal, #siteinfoCredits, #siteinfoStatus, #siteinfoIP, .center, .cartRemoveItemDisplay, .cartQuantityUpdate, .cartQuantity, .cartTotalsDisplay, #cartBoxGVBalance, .leftBoxHeading, .centerBoxHeading, .rightBoxHeading, .productListing-data, .accountQuantityDisplay, .ratingRow, label#textAreaReviews, #productMainImage, #reviewsInfoDefaultProductImage, #productReviewsDefaultProductImage, #reviewWriteMainImage, .centerBoxContents, .specialsListBoxContents, .categoryListBoxContents, .additionalImages, .centerBoxContentsSpecials, .centerBoxContentsAlsoPurch, .centerBoxContentsFeatured, .centerBoxContentsNew, .gvBal, .attribImg
{
text-align: center;
}
and right under it add aditional definitions for the .productListing-data
so you can add
.productListing-data {
border-bottom: 1px green solid;
padding: 15px 0px;
}
or what ever other combination of line color, thickness, style and padding you want.
hope this helps