I know that RAM, optical drive, and HDD are upgradable. How can I tell if anything else is upgradable as well? Will it be...

I got an Asus ROG GL552VW from Fry's for $480 about 7 months ago. It was a display model and was missing the Y key. Otherwise it was just fine. I have since learned that periodically browsing their website for non new stuff can be a way to save lots of money. Specially if a display item is somehow blemished (or missing a Y key, lol) you can bargain with them since all display stuff usually sells in store only. Since everything is still new in essence, you can usually get all repairs at no cost or for a flat fee via warranty.