Walmart has 500GB Sony Playstation 3 Console (3000346) for $149. Shipping is free, otherwise free store pickup is available if stock permits. Thanks Local2k
But if you MUST keep it horizontal, either due to space constraints or just personal preference, then I'd suggest putting something UNDER the system to improve airflow around the system.
I myself have my current PS3 up on a wire mesh basket like you would see in an office setting for an inbox/outbox for work. It has rubber feet on all four corners and that is what I have the PS3 balanced on so it's not even sitting right on the metal basket.
But anything that works to get the system up off the table or entertainment center you may have it in or on should help.