Hey, on caching... I cache a Velociraptor 300GB HLFS (2008 Design). It is amazing and there are only a few things I can tell the difference between when I had everything on the SSD and now. I am caching with a Crucial RealSSD 64GB.
Now when it comes to those new MSATA card. They where originally made with extreme pc enthusiasts in mind. Because of this many of them do not use all of the resources availible to max out the speed. I don't know how much money it saves them to do this but they do it. I can not recall if it was lanes or nands or something like that but the bottom line is many of them are not taking full advantage so I would suspect it possible that you end up with a underperforming MSATA drive. Unless you specifically find out what it is that I am referring to and ensure that the MSATA drive you purchase does not have that problem.
As far as putting everything on your SSD. There are a ton of people out there that will tell you that you should do it. Maybe they are correct and it is only my opinion but I feel that partition that I put my games on (usually C) needs to be atleast 250GB free after windows is installed so a 300GB HD. That's the minimum, so a 256GB or large SSD is a lot of cash. Once I re-installed windows on my velociraptor and set up the intel caching I did not miss having the OS and a few games on my SSD at all. Now I just install whatever I want and Intel decides what needs to be accelerated.
Hope this helps
Also shouldn't have a problem getting a 60GB HD for $30-40 new or used. You could probably find a better deal on a new one after rebate than on a used one as people are usually asking 80-150% of retail not worth it in my book for something that degrades (at least in theory, mines almost 3 yrs old and I have noticed no degradation after probably 15 OS re-installs and being use on a macbook pro and then my gaming rig).
READ THIS... Touches on performance comparisons and explains that you may want to ensure you are buying an SLC drive and not an MLC drive.
Also I am fairly certain that you can use the cache drive with an already set up raid system. In fact the way you set it up is you ensure that you drive is running in raid mode (doesn't need to be a raid array just on the intel raid controller) install windows as you would with the raid controller and then once in windows you install the intel software for the caching. I am fairly certain it works with any type of raid array.
Also you did not say what size your drives are but if they are 500GB to 1TB and you do not need that much storage you can "short-stroke" them which means to force them into thinking they are smaller which limits the amount of travel the read/write heads move across the platter. This increases response time and it also ensures that you do not write to the slower half of your physical drive since speed and response times suffer greatly on the outter half of the drive. I think this is essentially what a Velociraptor is in a way. They are taking the density of larger hard drives but using a platter that is much smaller with a very short travel distance for the heads.
GOOD LUCK