YES, you'll be fine.
I have a Nakamichi head unit feeding my oem amp, which in turn powers all the speakers just fine. Apparently the voltage of the Nak's speaker outputs work well as a signal to feed the oem amp.
All you need is your new Nak deck, a wiring harness (I recomend Metra, purchased at your local car audio shop) and an antenna adaptor. Install is easy as cake, and you will be quite pleased with the result, especially if you "think" you are satisfied with the current sound.