Can imac G3 DV 400 be booted when PRAM battery is empty?
Based on the link below, you will need to replace the PRAM battery to boot the iMac. You could always extract the HD, but then would have to find the correct interface to hook it up to your current computer.

http://www.mercenary.ca/articles/fix_imac_g3_wont_start_problem.php

There are a few videos on youtube describing PRAM battery replacement. One source for a replacement battery is http://www.newertech.com/products/pram_3_6v.php