Does SSD improve OS or everything?

Does installing an OS on an SSD + using a regular HDD improve only the windows bootup time or improve overall performance?

Also, how would you even set up an SSD-OS + HDD-storage configuration? Would the desktop be on the SSD or the HDD, and would you even have to install an OS on the HDD if you wanted your desktop to be on the HDD instead of the SSD if possible)?

Sub,

How would you install the O.S. on a SSD thats only 80GB and the Games and Programs on a raid array?

Well, at least in my case, because I have more than 80GB of games and programs.

As a related note:

I'm now using a setup where my main NAS is using SSDs as cache device (actually a RAID0 of SSDs). This works like follows:

A large RAID5 array with regular HDDs (WD Greens) which stores alot of data. Normally this would be slow for random disk access. But because i'm using a ZFS setup its possible to add 'cache' devices to this array, which act as buffercache (60GB) for the most used data. So the parts of this multi-terabyte array which i use the most, will be stored on the cache device instead which has very high read IOps rate.

Normally the RAM is capable of doing this, but the RAM is usually not 60GB large. As SSDs are also perfectly capable of doing random reads, this would speed up the portions of the array the i use the most. And best of all: ZFS keeps track of what i actually use. And even with large 10GB+ games its likely that only portions of the data files are used; ZFS also keeps track of this. Its hard to do benchmarks as the RAM caches too, but as i play World of Warcraft from this array it loads very fast; while normally on 5400rpm WD Green disks it would be kind of slow.

Of course, such a thing is only possible with a NAS capable of running ZFS. But it shows a solution to the problem of having more data/applications/programs than you can store on the SSD.