You can go as big as you want to go. Most people used to choose 200GB as this used to be the "sweet spot" for pricing, but that has now been replaced with 250GB and probably soon maybe 300GB.
Gotchas? That depends on how much research and reading you have done. I'm assuming that you have gone to the oztivo website and read the basics on how to image hard disks and what you require to get a TiVo up and running? If not then that is your starting point. My site (link in my signature) may be of some assistance too with some of the basic questions.
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