Mine is sitting in the cupboard (doors closed) and it's fine there were no problem with overheating or anything similar (I've no HDD attached, I was trying to run a neutrino type hub)
I've bought this copper heat sink, it's for the Pi and it was really easy to stick it onto the chips... it's about 2 EUR, free shipping, so it's not a big deal...
I think you should have some cooling because of the SATA HDD rather than the PI