Stray cat I want to bring in - best procedure?

Hi guys -
I currently own a cat who I've had for a little under 2 years. To the
best of my knowledge she's about 3 years old now. I live in a pretty
small apartment, 6-700 sq. feet and I work 9-5 so I try to leave lots
of toys around for kitty to play with and make sure the windows are
open so she can sit on the sills and bask in the sunlight and survey
her domain.
She really doesn't play with the toys, and I am beginning to feel that
she is very bored in her surroundings.

Recently, there's been a stray coming around that is very sweet. He
seems to have been someone's cat as he understands indoors and it
appears he knows how to use a litter box. I don't plan on bringing him
in until I could take him to the vet and get him thoroughly checked
out though.

Problem is, whenever there's another cat outside, my cat goes wild.
She seems to have calmed down a slight bit with this cat hanging
around quite often, but they still hiss and moan at each other through
the screen door. I want to do this stray a favor and give him a home,
but I don't want to inadvertently stress my cat out MORE by trying to
get her a playmate.

I've read up on acclimating cats to each other, and there are two
"rooms" in my apartment that I could use to keep them separated for a
week or so and while I'm at work until I'm home to supervise
encounters.. but this might make my current cat freak out.

I'm also worried about the smallness of my apartment. What should I
do? Take him to the vet, check him out and then make a decision? Or
should I just take him to the pound so he at least has a chance that
someone will find him a loving home?