Should I upgrade or buy a new rig?

So as stated in the title, I am not sure if I should just get a new rig or upgrade what I currently have.

Budget:
[maybe %600-700 max, im poor right now so sad :( ]

Expectations:
[Light - medium gaming, and watching youtube/twitch ]

Usage:
[I want to play recent MMO games such as wildstar/archeage and more of the sort, but it doesn't have to be high or ultra graphics, I am totally fine with medium graphics. As well as the usual internet surfing, youtube/twitch, and all that good stuff]

Special Needs:
[not really any special needs, I don't mind a loud system, It does get kind of hot where I live so I guess I would be a little worried about my computer overheating and such]

Saved Ticket #: [Please Type Here] um no specifics here but I was looking at the Vanquish 2 series

Specifications: This was my first DYI desktop that I built like a few years ago so I am pretty new to building it myself. Thanks for your time.
[Current setup is like pretty outdated:
Antec 900 V2 case
AMD Phenom II x2 555 BE ~3.2 GHz
Asus m4a785td-v evo
4gigs G.Skill RAM
Nvidia GeForce 8600 GTS
550 Rocketfish PSU

(its so old, dont make fun plz lol)]

Thank you for your reply bprat22, and Omg I forgot. My monitor resolution is like 1680x1050, 22".
It's an old monitor too lol