In case you wonder — like I did — to employ foobar2000 (fb2k) to handle the lack of ReplayGain (RG) info in a file’s tags nicely so it doesn’t blast away your eardrum I have a set of very usefull links. At hydrogenaudio.org I found the Intermediate User Guide for fb2k explaining, among others, the options one has setting up fb2k for ReplayGain. Most importantly one should slide the bar in the playback preferences pane for “without RG info” to a value that reflects the average sound level of all tracks. This, of course, would mean to scan all your files. For my couple’o weeks worth of playback time fb2k estimates just over 24h for that job. So it was an easy choise to just stick to the suggested value -8db.
Last but not least, I will mention the two preamps in the Playback preferences. Except if you know exactly what you are doing, it is not recommended to raise the output of the preamps above the default 0.0dB in any way. However you can use these to slightly compensate for the difference between replaygained and unreplaigained Tracks. Simply estimate your average Replaygain level and lower the preamp for files without Replaygain info by that value. I found -8dB to work quite well for me. This obviously should not be used to compensate for not properly Replaygaining your tracks, but definetaly will protect your ears and your equipment when coming across tracks that miss Replaygain info.
Secondly there is a more detailed description on the Playback settings in the same wiki. Besides some mathematics on RG it also points out some interesting knowlege about pre-buffering and DSP settings