<p>Will IE6 negotiate a 256 bit AES SSL connection if the server is capable?</p>

Possibly, I can't work out for sure yet, but what I can tell you is AES256 appears to be restricted by US Export restrictions on high-security cryptography, and for this reason, some platforms may lack this capacity.
Also, upon further searching, msdn pages ( such as
this one
) seem to point to AES support ( period! ) being only available since Vista / IE7.
No news yet whether or not MS decided to backport it, but it looks dubious.