The 32-bit Windows 7 Beta was leaked about two weeks ago and yesterday the 64-bit version hit torrent sites. The file is named "7000.0.081212-1400_client_en-us_Ultimate-GB1CULXFRE_EN_DVD.ISO" and weighs in 3.15GB.
If the 64-bit build is anything like the 32-bit, performance should be quite high. Keep in mind though, build 7000 has an MP3 corruption bug, though a patch should be released any day now. This second leak is yet another slap in the face to testers who have been patiently for something to test (although select testers have received early builds), though the good news is that they won't have to deal with the headache of corrupted MP3 files once they get Beta 1.