All windows versions since NT 3.1 ( on which Win2k and all subsequent releases have been based ), have absolutely used availible free ram to cache disk IO. Journaling filesystems like NTFS were designed to ensure that power loss would not cause catastrophe using techniques such as a journal. Such writes bypass any cache on the disk since the disk cache is also lost in the event of a power failure.
Disk write cache does not help since games do not write much data in the first place. Any data they do write certainly can fit in the kernel filesystem cache, which generally has access to far more memory than the on disk cache, thus, adding more disk cache will not help ( unless you add a LOT of disk cache ).
Yes, it is a generalization that games are smart enough to preload data they will need before they need it. It also happens to be largely true, and adding disk cache will not help games that do not, since cache mostly keeps around data that has been recently acceded, not data that will soon be needed, but has not been accesedd lately, thus a badly written game engine will not benefit from more disk cache.
If you think it is easy to add cache to a drive, it is even easier to add ram to the system, and let the OS use it for cache.
Extra cache on the drive would not allow the drive to delay background writes any more than the OS could given the same amount of additional ram, hence, it is better to add the ram to the OS than to the drive. If unzipping in one process slows down another process that is just reading that much, then that points to a fault in the OS, not the drive; the OS should be prioritizing the reads over the writes ( and Linux does so ).
In terms of data safety, any application that cares about safety bypasses the OS and disk cache and makes sure its important data has hit the physical medium before proceeding anyhow. Moving cache from the OS to the disk theoretically could allow applications to only require the data hit the disk cache before proceeding, but applications are not written to do so; if they care about the data, they make sure it is actually on the disk, not just in the disk cache, because they care about power failures, not just kernel crashes.
