This
site and this
site are a gold mine of information on iChat and the latter one especially has detailed instructions on how to set-up iChat to PC AIM videochats - IIRC it is from a guy who provides a lot of user support on iChat at Apple Discussions.
From the first link, this is a brief list of things that are needed on the PC side of things:
I do video chat with many PCs, and a few of my regular chat buddies use PCs. In the cases where I have encountered difficulty, the most frequent problems chatting with PCs seem to be:
-PC's broadband speed too slow, (Check your Broadband Speed)
-PC needs to use Windows XP and AIM 5.9,
-PC needs to install the "extra" Windows XP file to support A/V,
-PC needs to "tune" his camera/mic,
-PC needs to open his XP software Firewall,
-PC needs to set router ports IAW AIM FAQ.
Another site you may be interested in that I recently discovered is
Mac Messenger Guide which provides information on alternatives to iChat. For instance aMSN and Mercury Messenger both support video-conferencing with MSN on the PC (I haven't had a chance to try either yet so can't comment on the quality). From the description, Mercury Messenger offers feature parity with MSN Messenger 7 on the PC.
HTH.