Im running windows 7 x64 Build 7000 on the same machine that I ran all the x64 versions of vista on, heres the specs, for the faint of heart... please look away now:
AMD Turion x64 ML-42 processor
1g NON-ddr ram
100g HDD
ATI Radeon Express 200m video (128vid mem)


It really runs great, I need to add another gig of ram to the laptop, im usually running at 60-80% ram full. Boot time is usually about 1min from welcome screen to ready-to-run. I have only a audio and cardreader issue, but as for everything else it runs like a champ... with one leg. :)