Seeking opinions from people who have cats

So we are thinking about getting a cat (we've not thought much further than this, but then we wouldn't be able to do anything about it until mid September, when we get back from the States).

I would like a cat, but I am hesitating, largely because I quite like our local wildlife population (we have a lot of small birds, including a regular visit from a willow warbler, lots of frogs, and a lot of voles/ fieldmice). And, obviously, cats will do what cats do.

Someone suggested keeping the cat inside, and I know that some people do do this, but I am not entirely sure that's kind to the cat. We have a big house (four bedrooms and a study), but of course that's not remotely the same as having freedom to wander about outside, and I worry the cat would get bored/ frustrated.

What do you, oh cat-owning/ loving flisties think? I may end up concluding that I can't cope with the prospect of the cat cutting a swathe through the local wildlife, but I would rather just not have one than keep it inside all the time, if that would be bad for the cat...