The build is 7100 and carries the build tag 7100.0.winmain_win7rc.090421-1700. Windows 7 brings many new features, compatibility improvements and speed improvements over that of Windows Vista.
Expect to see an announcement from Microsoft about the RC of Windows 7 on May 5th, with a public release somewhere around the 9th.
At the moment there are some torrents purporting to be build 7100 . They are fakes so dont bother downloading for now