Why not simply buy a normally-sized SSD to replace your HDD with? Or just buy a small mSATA (smaller than 240GB or even smaller than 120GB) for caching purposes - there should be an option in laptop's BIOS to enable "Smart Response" or something like this...
If you really want to have Win8 installed on mSATA drive - buy it, insert it to its slot (you might need to use a couple of screws), then go to laptop's BIOS and set it as primary bootable device (or whatever it will be called) and then just format the current HDD and install Win8 to your new mSATA... You just need installation media for fresh install. Also, make sure that whatever mSATA you're buying is compatible with your laptop model (or buy it at a place where you can easily return it if it won't be compatible).
The SSD is for the OS/games/programs/HD porn. The HDD is for media storage/backups. I'm also not rich enough to afford a 480GB+ SSD
I don't think it would be worth the money I would need to get a >500GB one. My current 500GB HDD is almost full.
I'm pretty familiar with working on computers in general (worked computer repair during college), but I haven't been able to keep up with SSDs or windows 8. I'm mostly concerned with any special considerations that need to be made with SSDs/windows 8 vs HDDs/older versions of windows.
Consult your laptop hardware maintenance manual (usually available at the manufacturer's site) for how to access the mSATA slot.
Once you install the mSATA drive, you can install windows by
a. cloning the partition over from your laptop (various tools that can do this)
b. running the system recovery bootloader (if you have one) on the HDD and selecting the SSD drive as the target
c. downloading a windows 8 ISO onto a CD or USB drive and installing it on the SSD with your existing key
You can select the SSD as the boot drive in BIOS.
I've done both option b. and c. on my wife's laptop (128 GB SSD and 500 GB HDD) and option a. on my desktop. The Liteon drive is a pretty decent drive and similar to the plextor m3 in my desktop and the crucial m4 in the wife's laptop. Good luck!
With regards to option C, I've read that the key is built into the BIOS or something like that. Is that true?
Also, how reliable are SSDs now? I know a few years ago there were a lot of problems with SSDs, particularly ones from OCZ.