RAM / HDD (or SSD) are the easiest.
CPU and GPU can both be removed, but are more complicated.
CPU wouldn't really be worth upgrading as mobile chips are significantly more expensive, and your CPU should be fine.
The GPU...is kind of a toss-up as to what it can support. The BIOS (and vBIOS on the GPU) is the limiting part here. NotebookReview is my go-to to see what kind of mods / upgrades can be done on gaming notebooks. According to this thread:
MSI GT60 0ND-250US GPU upgrade | NotebookReview
Your options for GPU upgrade are the 780m or 680m. However I don't know if that requires you to mod / change out the cooling as well (some models require you to do that). Same with any vBIOS / BIOS mods that may be required - and if so, you'll have to install modded drivers for everything to work correctly. Also keep in mind that mobile GPU's are much higher priced than desktop counterparts, so you'll be dropping some decent cash on a GPU upgrade that may not offer that significant of a jump in framerate for gaming.