Aaah, our local Gentoo zealot returns! :)
So the tradeoffs are - a distribution that is unbeatably fast, slow to setup but teaches you everything about the insides vs. a slightly slower system - thats quick to setup but doesn't teach you much at all. Try Gentoo first ;).
While I don't have any first hand experience to offer, Fedora Core 3 and Ubuntu both offer native amd64 versions, and they are flavour of the month with wlug members at the moment. If you are familiar with Debian, (I believe you to be a WAND student?) Ubuntu might well be your best answer. A couple of people (Perry springs to mind) run Gentoo on an AMD64, but have had problems running the ~amd64 branch due to things not being entirely 64-bit ready or clean. Your mileage may vary as to what does and doesn't work, but come this time next year, the answer should be an emphatic 'everything'. Craig