How important is domain name in SEO?

i will answer your question in a different way ... do not mix themes at your site ... mixing themes is a big no-no

this means that if your site has nothing to do with baseball and you add a baseball folder + content, then your rankings will suffer

if on the other hand, your site is about baseball, i would opt for the first one with a link to that folder on your home page ... my understanding is that the second one may be considered as a separated domain name which would mean having to promote it separately

perhaps others have something to add

-SearchSpiderz