Service Pack 2 Release Candidate for Windows Vista and Windows Server 2008 are now available for download to TechNet and MSDN subscribers to test prior to final release. SP2 for Windows Vista and Windows Server 2008 include updates that have been delivered since the release of SP1, as well as support for new types of hardware and emerging standards. SP2 weighs in at a sizable 302MB for x86 systems and 508MB for 64-bit operating system versions of Windows Vista.
Having installed Windows Vista SP1, the probablity of you removing it is rather bleak. However, when you install SP1, Vista gives you the provision of removing it, in case you choose to do so in the past. This is done, by keeping the old files that were replaced by the SP1 update.