Some facts:
- English in general is an overloaded field.
- Extra certifications always give one more options.
My degree in English never got me a teaching job; however, my certification did. NCLB is really putting the pressure on schools to hire the most highly qualified teachers--and that even trickles into private schools. That said, there are a TON of independent schools in and around D.C., and with the experience you already have, you might fit in just fine.
Or you could end up waiting tables, substitute teaching, and writing freelance. (Not trying to be snarky, but that's what I ended up doing with my English degree!). Once I got my certification, a whole lot more doors opened up for me. I work in a private school in D.C. now. We do require certification.