Windows ALREADY caches a great deal, which is why it's such a memory hog. For example, Windows caches directories once they've been accessed; no need to read the filenames and sizes, etc., again when you go back to them. Files being written to are cached and only updated when the buffer gets full, to prevent having to access the same parts of the disk over and over (which would lead to latency and "drive chattering"; this has been somewhat moved to hardware by NCQ (Native Command Queueing), moving the buffer over to the hard drive, but Windows still does it.)
Windows has been doing this since the Windows days - look up "SmartDrv."
The Free Edition (limited to Windows 32-bit Win2000 / XP / 2003) is able to use 'invisible' RAM in the to 4 GB 'gap' (if your motherboard has i946 or above chipset) & is also capable of 'saving to hard disk on power down' (so, in theory, allows you to use the RAM disk for Windows XP swap file and survive over a 'Hibernate'). Whilst the free edition allows multiple RAM disk drives to be set up, the total of all drives is limited to 4096 MB. The current version, VSuite Ramdisk II, has been rebranded as 'Primo Ramdisk', all versions of which are chargeable.