The first service pack for Windows Vista will not be released until early in 2008, but from what we can see, the update is more about stability and security than performance.

Many alterations will not be obvious to the regular user.  In fact, you probably will not notice any interface changes at all.  The main improvements focus on performance and reliability when reactiviating a machine from the Hibernate and Suspend modes.  There will also be enhanced driver support (thank you!), and increased security.

According to Microsoft, typical load times for the final version should range from 30 to 60 minutes.  The installation requires 7GB of free hard drive space, but the install file itself will be about a 50MB download.

After the update, you should notice shorter boots (by about 20 seconds), faster copying and extracting, more efficient use of memory.