ok you have to have a little faith in apple here, in that they wouldnt just ship out powerbooks that would overheat so easily.
firstly, theres no way your firewire port is going to "overheat" by moving the small amount of bandwidth that comes through your internet connection, through to your firewire drive over a long period of time. infact moving maximum bandwidth through firewire over a long period of time still wont make anything overheat. if it did, then youd need to send you powerbook in to get fixed.
secondly, special cooling pads and raising your laptop up above your desktop are, in most cases, definitely not necessities, even if your doing cpu intensive stuff or leaving your laptop on for a long period of time. heat will not build up inside the laptop over time; thats what the fans, heatsinks, and heat dissipation design are for.
thirdly, claiming that the higher mhz your powerbook is, the hotter it will get, is a false statement in many respects. you have to take into account that newer revisions have better ways of getting rid of heat. my friend and past roommate has an 800mhz powerbook, and i have a 1ghz (both 15" Ti), and his definitely runs a lot hotter than mine does.
my powerbook sits on a wood surface desk, with no extra cooling peripherals, and ive never had problems with overheating. and i do plenty of cpu intensive tasks, often leaving my computer on for extended periods of time also. so i say go ahead and get the laptop of your choice and dont worry too much about overheating, because apple worries about all the heat issues during design, so we dont have to.
