Maybe it will benefit you having a 64 bit CPU to go gentoo (But it sure as hell can't compete with slack in any other way :P )
Suse was the first firm to officially support 64 processing, so it's likely they still hold a lead ? Anyway Suse should be able to run really really well on your CPU. I'd go for suse if I were you, ESPECIALLY if you're relatively new.. Suse my friend :P
Anyway if you prefer trying it out before buying... perhaps some p2p or torrents will allow you to do just that. Anyway I think SUSE has some option to download the whole thing during installation too (legal hehe
)
After the initial learning if/when you feel like moving on, contact the maintainer of slackware to make him add 64bit cpu support in the next version :P Personally I have never had as lovely a distro as slack.. Tried:
Red Hat (7.2, 8.0)
Fedora (core 1)
Mandrake (8.0, 8.2, 9.0, 9.1, 9.2 (+ rc's etc))
Gentoo (1.4 I believe)
Gentoox (gentoo for the xbox.. Now that was unpleasant, the xbox doesn't have the power to compile things in a reasonable speed, and the HDD makes alot of noise... eww)
Mandrake 9.1 (also for the xbox)
Debian Linux (uhm... woody)
Libranet Linux (debian with extra's, more userfriendly)
Slackware 9.0 (yay! :P )
Of course many has tried more than me, it's more to support my pro-slack views :P
Sorry to lead the topic a bit off but...
<rant>
Chase missing tarballs ? Are you nuts ? Chasing is something we know from RH/Fedora/Mandrake not slack! As mentioned we have swaret and others. Even so it's so easy to install packages with the nothing less than brilliant tgz solution.! Compiling tarballs have never been so easy. Look at the list above. I can tell you right here and now that on any of these systems, I've never had as easy compiles as slack. I used to dread tar's, now I like them cause they give me the optimal choice (control freak) while being fast and truly optimised for my system. I'm sorry but how can packages for gentoo be optimised if they all by default uses the same compile flags ? Running slack is hard at first (yes!) but in the end. I doubt you could move away again even if you wanted.</rant>