I think you are talking about two different things... in Windows Vista ReadyBoost lets you use a fast flash drive for ram... at times improving application performance... from the sounds of it you want to use a fast flash drive for hard drive space... giving you faster access to your files.
This you could do with WHS (or any other Windows PC)... only it’d be extraordinarily expensive (~$8-$25 per gigabyte) to do so.
Off hand I do not recall the minimum disk size for adding a drive to the storage pool... however I believe that a 2 gig flash stick would be below that... one way with flash to get above that limit would be with a SSD (Solid-State Drive) or two... which is nothing but a PATA or SATA hard drive with ridiculous amounts of flash memory inside.
Be warned though… as fast and nifty as SSDs are... you pay through the nose for em... paying ~$300-800 for a 32 gig unit... all the way up to ~3200 for a 128 gig unit.
