I think it is possible to carve out a partition to be used as a cache. Bit it is usually recommended to give the "C" boot drive all of the space. That way, you can cache writes also.
If you want to cache a hard drive, get a ssd specifically designed for the job. An Intel 310 20gb unit I believe.
A SSD will come with some space reserved and unavailable to you. A 96gb ssd may be advertised as a 80gb unit, like the Intel X25-M 80gb . Some ssd vendors may reserve less space. Performance is affected on a fullish ssd when updating. Exactly how differs by the algorithms the vendors use. My take is to not worry about it too much. When the drive starts getting too full, you are going to change it out for one with more space anyway. I think that point will be about 85% full.
I installed windows 7 32 bit on a 40gb drive, in a laptop, along with ms security essentials. As I recall, it took 15gb without any attempt at reducing the space. I added a couple of games for travelling without problem.
In general, though, I think 60-80gb would be a better minimum.
If you are using a SSD for the OS drive then you don't really need a small SSD to use as a cache , you are already using a SSD so the speeds are fast as it is. You would get better results if you had a conventional hdd with your OS on it and then had a small SSD to use as the cache. In using a SSD as the C drive it would be good to have one that was 90gb or 120gb , it would give you enough room for the OS a few games and the updates that will come out for Windows , the games and the drivers. You would be surprised how fast it fills up , you don't notice it as nuch with a conventional hdd because it has so much space but when you are dealing with 100gb or less space it fills up fast so you want to have as gig as you can afford.You don't want to go past 75% capacity on the SSD as performance will lessen if you go past that amount.