Windows 7 32bit will fit and surprising but true, Dabs here sells a tablet PC with a 16GB SSD running Windows 7 and Android. If 20GB seems a little small, I sure wonder how customers of this product will get on with 16GB, not to mention the space taken up by Android.
For my own curiosity, I have carried out a clean installation of Windows 7 Home Premium 32-bit in Virtual PC, installed service pack 1, IE9 and all the patches up to date. To reduce disk usage as much as possible, I disabled the page file, ran the Windows disk cleanup utility and disabled system restore:
With the user documents, pictures, etc. folders redirected to a HDD, that would leave over 10GB of free space for future updates, applications, etc., so a 20GB OS drive is completely useable, but something which will become cramped over time. My current near 2-year old Windows 7 64-bit OS drive is using 45GB of space, so a 20GB or even a 32GB SSD would have reached its capacity a long time ago.
However, as a cache drive, I'd imagine this will be very effective, especially going by the reviews I've seen so far of Z68 boards. Unlike the Seagate hybrid HDD, this SSD is used to cache write operations as well. 20GB of cache should be plenty for most users, since even if the OS grows to over 100GB, chances are that less than 20GB of that data would be read no more than once in a month. It's worth noting that the motherboard chooses which data to store on the SSD for fast reading, unlike an SSD used as an OS drive where it is up to the user to decide what to put or install on the SSD.