I am a single mom to an almost 12 yr old boy, and i really really regret not having another one sooner. I say go for it!
I'm adopting a six month old baby (i have had him since three weeks of age, he is my foster child), so there is also the adoption option for you. But the age gap is so large that my son is still basically an only child, he doesnt have a same age "playmate"....i still plan on adopting a boy closer to his age (actually i want about five kids total!) You could also adopt a child a bit older, closer to your child's age, if that was something you are interested in (and if you adopt from the state, you often get a montly support stipend and medicaid card as well)...if you want more info on that, pop over to the adoption forum, we'd be happy to help!
If you want a baby now, go for it!
Katherine, single homeschooling mom to Boy Genius (17) Thing One (6) and Thing Two (6) and one outgoing Girl (12) and hoping for more through foster care and adoption