By default, the Ram preiew resolution is adaptative and not full quality, as it's just to "preview". You can switch this behavior to full quality, and by default, when you render, it will always be "Full quality" before being compressed by your output module.
Hope that helps,
Also keep in mind, that for several standard video formats, AE will separate fields, which will result in only one field being displayed, reducing the vertical resolution. Disabling it in the footage interpretation and doubling the framerate just so all fields are displayed correctly may be necessary, e.g. for masking. If you're just converting stuff, I think you don't need to worry - with the right output settings, the sampling will be correct.
Thanks for the help guys, my problem is solved. Thanks!