I need advice on how to go about running Linux within a predominantly Windows environment

I really, really want to start running Linux within my network because, well, I just want to. I love Linux (and Unix, Solaris to be exact) the few times I've had a chance to work with it. So, what would be the best way to go about introducing a Linux server within an Active Directory network? We do not host our own web server so running an Apache on a Fedora system or something wouldn't be an option. I work for a not for profit organization that has limited funds and I would like to get a dedicated hardware firewall on the network. So, I was thinking about perhaps setting up a Dell Poweredge box we have that is not being used, installing Linux on it and using that as our dedicated firewall which would be much cheaper than purchasing some brand name appliance like SonicWall or something. How does that sound? Thanks everyone.