For curiosity, I downloaded Windows Vista Home Premium 32-bit with service pack 2 and the same without any service pack from Technet (I've a subscription.) I created two virtual PCs, both with 1GB of RAM and installed the edition without any service pack in one Virtual PC and the version with service pack 2 in the other.
The Virtual PC with Vista service pack 2 took about 15 minutes to install the OS and about an hour to download and install all the patches up to date, with about half this time just downloading the patches (I've a 3Mb connection.)
With the other virtual PC, it also took about 15 minutes to install the OS. It then took at least 4 hours to download and install service pack 1 & 2 as well as all the patches up to date. With this installation, most of the was spent between installing service pack 1 & 2.
Despite both Virtual PCs set up the same way apart from the OS, the Vista OS that was installed starting without a service pack felt a lot more sluggish and seemed to spend a lot of time thrashing the hard disk, even after installing service pack 2. Booting up and shutting down also takes longer than the other installation.
Here's the RAM and HDD usage of both installations. For the RAM/PF usage, the OS was booted and left idle at the desktop for a few minutes before taking the readings:
Windows Vista Home Premium with Service pack 2 installed - Then patches applied up to date, including IE8:
- Memory usage - 320MB
- Page file usage - 419MB
- Disk space usage - 14.4GB
Windows Vista Home Premium installed - Then applied service packs 1 & 2 and all patches up to date, including IE8:
- Memory usage - 381MB
- Page file usage - 403MB
- Disk space usage - 30.8GB
These values will likely be different for Vista 64-bit. Also, these are obviously under ideal conditions, since I haven't installed any additional drivers, software or Antivirus package, so these figures are purely from OS usage.
So for less RAM usage, a much less bloated OS installation (based on HDD usage) and a better performing OS, I would highly recommend slipstreaming Service Pack 2 into the Vista OS installation before installing Windows Vista. This will also save a few hours installing service packs 1 & 2.