QUOTE(Killmaster @ Sep 9 2003, 08:54 AM)
Could the replaygain setting be applied to the file in question while it's playing? I tend to do it as I listen to them and would rather not have to mess with once it's done.
Yes I've just tested it (and you could have to

)
Basically there is a queuing system on file updates (except for renaming) so that, if you attempt to change the tags or headers (from within foobar2000) while playing, it waits until you have finished playing that song and then does the update.
You can confirm this by playing a file, ReplayGaining it (while playing) and then open the console (Components->Show console) and you will see
INFO (CORE) : file info update pending / file busy on :
<name of file>
Also when you play another song and then look at the file info of the one you ReplayGained, it will now have the RG info in the header as you want.