one more thought...
If you're considering the PMD-661 vs. Sony PCM D 50, sound quality between these two machines is _probably_ fairly similar. I admit that I'm only guessing here, but it's likely that microphones and their placement will make more of a difference in your final recording than the sound quality of the boxes. Their features and external connections are different, you can pick the set that fits your interests best. If you have a definite interest in using external microphones, the Marantz or the Fostex might work well for you. XLR connectors and phantom power are a big plus over a miniplug and plug-in-power.