Cloaking- You MUST have this if you expect to get good nukes off, now i'm sure most of you already know this but i'm telling this too any people who are not familiar with this. Even if you aren't gonna nuke anything its still important for spying or harassing.
Ocular Implants- This upgrade increases the sight range of the ghost for nuking and sight (i'm not sure about attack range). This is highly recomended therefore making it mandotory because it will keep you out of range of cannons/turrets/spores and any other detectors a lot better.
Extra Energy- This is mostly important for ppl who like to hide their ghosts in ppls bases cloaked and/or for ppl who use lockdown while nuking.
Lockdown- Launches a missile from the ghosts gun that encases an enemies unit in a white bubble therefore immoblizing him and he cannot attack or move and all special attributes to that unit are disabled i.e detector.