I am using Windows 2000 just now. I found a 12GB partiton plenty for years but gradual increase in usage due to email, monthly updates from microsoft, the odd little utility added here and there, and significantly all the same games from Oblivion and other games stored in "My Documents" eventually ran me out of space. 24Gb is massively plenty for me for the forseeable future.
I'm not so sure exactly how much space XP will need. That will be my next OS upgrade in the next year or so on this PC. I have used XP a lot since it came out and have a gut feeling it isn't so different from Win 2000 in it's size and speed of growth. I have no doubt Vista is a bloated monster and I wouldn't like to comment on how big it is after loading and how fast it grows.
Ask that question again slowly, out load, remembering not so many years ago and in XP's lifetime we were using 40GB hard drives. "Is 24GB enough for the OS"
Hard Drives don't fill up. Partitions on a hard drive fill up. You have to put at least one partiton on any hard drive before an OS can use it. The thing is OS installs tend to push you down the route of one physical hard drive equals one partiton, and at the same time tend to hide the partition creation from you. I just like to use the data structure that is the "partiton" to top level organize my files on my computer.
In terms of performance I think that having the OS on a different partiton AND on a different hard drive AND ideally in a perfect world on a different controller from your commonly used apps and data would increase performance. Getting the swap file off the system partiton and onto a different drive is useful too. a lot of people use a separate very fast relatively small comparatively expensive hard drive for just their OS for this very reason.
In order to make an informed decison about this you need to think about how a hard drive works. When the drive has to read a piece of data, it has to move the read head to the correct track, then wait while the platter spins beneath it to bring the correct bit of disk holding the data round to it, then read the data off. This is mostly slow mechanical stuff. When a partiton starts to get full files get badly fragmented which means that there may be hundreds of fragments all over the disk to be read instead of one contiguous area. That slows things down somewhat. With frequently accessed stuff on two separate disks you can see that there should be less mechanical movement - the OS can be doing one thing on drive while data and apps are doing something else on another. Clearly this leads on to the various sorts of RAID implementations. Incidentally Hard Drive caches are simply an attempt to hold stuff in memory on the drive to prevent having to go through the slow electro mechanical read process. One further thought for you - if there are two two partitons on one drive the mechanicals of readin/writing to the two are the same as if there is just one big partition so there is no point for example putting the OS on the first partition and the swap file in the second partiton. To get the performance benefits you need to get two (or more) physical hard drives working simultaneously to replace one hard drive doing two (or more) things consecutively.
Hope my ramblings help.
