For the record, I got bored and rebuilt Apache and PHP to the following:
Gallery version = 2.0.1+ core 1.0.9
PHP version = 4.4.1 apache
Webserver = Apache/1.3.34 (Unix) PHP/4.4.1
Database = mysql 3.23.49
Toolkits = ArchiveUpload, Dcraw, Exif, ImageMagick, Thumbnail
Operating system = Linux mother.killernuts.org 2.4.18-3custom #3 Tue Aug 13 15:10:18 PDT 2002 i686
Browser = Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/412.7 (KHTML, like Gecko) Safari/412.5
and now the PGThemes install works - still don't know exactly what was causing Apache to crap out before, but a newer one bucks the problem, whatever it was.