<< The differences seem compelling in favor of Fedora Core:
1. Fedora Core is free-beer, price of $0, whereas SUSE is $767. 2. Fedora Core is GNU-free, whereas I'm not so sure about SUSE.
SuSE is GNU-free starting with 9.1 (the installation tool YaST is GPL in this release), the professional edition is $90 and should work on AMD64 (kernel 2.6), the release has gone gold and should start shipping end of April.
Fedora Core 2 is release is planned for May 17th, kernel 2.6 with SELinux on which needs lots of fixing to get right (progressing nicely though), test3 beta will be released on April 26th.
Fecora Core 1 is 2.4 based and I believe users of AMD64 are happy with it.
I don't know about debian and mandrake.
I've no AMD64 system yet, but since I just did a review of the Linux state sharing some information would make sense :).
FC1 looks like the distribution to try first with an update at end of may to a newer distribution if needed (2.6 is reported as sometimes much faster on servers than 2.4).
Hope this helps,
Laurent