In skin1/customer/main/buy_now.tpl
Add...
Code:
name="orderform_{$product.productid}_{$product.add_date}"
To the <form> tag... then the buttons will function.
eg.
Code:
<form name="orderform_{$product.productid}_{$product.add_date}" method="post" action="{if $product.is_product_options eq 'Y' && !$buynow_enabled}product.php?productid={$product.productid}{else}cart.php?mode=add{/if}">
As for the button styling, you've changed the doctype to transitional, which might affect the appearance. I think more urgent, is the layout doesn't look correct in IE6.