[CentOS] CentOS or other Linux Internet Router/Gateway

Hi Everyone,

I've got kids who are growing older and I want to build a Linux box to
filter Internet access. I've got six computers on the Internet, plus
the laptops -- most run Windows. I'm not sure if it's called a router
or gateway or...?  I don't mind doing the leg work, I just don't know
where to start. I'm pretty sure I'm *not* looking for a web server
(though it might be fun to have a local web) -- I might also use the
computer for a file server, but I mostly want a... proxy server? Not
sure what tool I need to use. I seem to remember there were specialty
Linux distributions for this purpose.

Once I do figure out what kind of box I'm building, would I still be
able to put my Linksys router behind it, or would the Linux box *be*
the router? -- and would I have to get a wireless switch instead?
Would a Pentium III handle this duty, or would it gum up the works and
make the Internet too slow?

Sorry for stupid questions. At some point I'm going to have to learn
about Linux servers. I'm hoping this project will help me get started
on that.

Thanks for any pointers.

-- 
RonB -- Using CentOS 5.5