Yesterday, Microsoft released a new Release Candidate (RC) build (6001-17042-071107-1618) of SP1 to testers. The latest build weighs in at 434.84MB for the x86 version and 734.3MB for the x64 version.
Testers who already have the previous SP1 beta installed, however, will run into somewhat of a roadblock when it comes to the new RC build.
"Windows Vista SP1 does not support build-to-build upgrades," states Microsoft. "Therefore, if you have installed a previously release build on your machine, you have to uninstall this old build before installing the next build of Windows Vista SP1."
Windows Vista SP1 RC is currently available for download from the Microsoft Connect website for current SP1 testers.