I recently moved to foobar2000 (i love it) and aac as well. I rip with EAC and encode with foobar using nencode/Nero. After the files are converted (wav->mp4)
I tag them with foobar as well. The last step which i'm confused about (bit of a noob) is replaygain. I select the entire album but which should I scan with: scan per-file track gain, scan selection as album, or scan selection as multiple albums using the album field?

I guess I would just like not to fiddle too much with the volume control (loud and soft songs).

Also, should I be scanning my entire collection at a time or is it alright if I just do an album at a time as I rip to achieve this aim?

scan per-file track gain - will calculate only track type of gain, also called radio gain.

scan selection as album, or scan selection as multiple albums using the album field - will calculate track and album gains, so it is the easiest to use the latest if your files are tagged allready.

difference:
http://replaygain.hydrogenaudio.org/radio.html

(i do wonder if there is a way to tell foobar to use radio type when playback is set to random and album when default?)

album at a time will get the same results.

I just add every album to the playlist, select all, then "scan selection as multiple albums using the album field". Seems to me to be the easiest option to me.

This doesn't work if you have many "Greatest Hits" albums by different artists. So all "Greatest Hits", "Best Of" and other that kind of albums should be Replaygained separately when using "scan using multiple albums"