Server :: Setting Up Harddrives On Fileserver For Maximum R/w Speed

I am currently in process of remodeling my home file server and would like some advice. The server has two internal hard drives that are rather small (10-15G) and I've now ordered a larger 2 TB drive which for the time being will have to run as an external drive through USB 1 (going to be rather slow).

I'm probably going to put the OS and swap on one of the internal drives but I was wondering if there was a good option for increasing the system's performance by making the second internal drive act as a kind of buffer for the 2TB usb drive. I'd like both the advantages of the large size of the USB drive and the fast read/write speed of the internal drive. Would it be possible to put it together so all reads and writes to the fileserver would first go to the fast internal drive (or even better, to internal drive and USB drive at once, although I suppose RAID is not an option with USB attached storage), and would then be put to the large drive. It would also be nice if the most used files from the large USB drive would be cached on the internal drive for fast read speed. I understood that ZFS would help me accomplish something the like but as I understand, it's not that easily available on Linux.

The current plan is to make the large drive a simple XFS drive and build a small daemon on the server that would simply move all new files from the internal drive to the USB one, once they are not used but it would be nice to have a more low level solution.