there are a few ways to cool a dm800,
just search ebay for netbook cooler or
cut the adapter off an old mobile phone charger and connect it to a 12v pc fan and pop it on top or
cut the end off an old usb cable and connect it to a 5v pc fan and either insert inside or drop on top (theres a usb port under the hdd caddy)
alternatively, if the fan is noisy, remove it, remove the sticker from the centre of the fan an put one drop of 3 in 1 oil on it, works a treat!