Thinking about upgrading part(s)

[Nairb4]

What would I benefit from the most?

I built this computer a little over a year ago with help from various forums / guides to help picking out parts (especially the guides here). I've been thinking about upgrading but I'm not sure what would help me the most.

EVGA 660ti
Intel i5 3570k
Asus P8Z77-V LK mobo
Cooler Master Hyper 212+ cpu cooler
Corsair Dominator Platinum 16gb DDR3
Thermaltake 430W PSU
Samsung 830 Series 128GB SSD
500GB HDD (can't remember kind, came out of previous PC build)
Cooler Master HAF 932 case
running dual 24" LCD monitors

I would like a bigger SSD as it's mostly taken up with Windows7, WoW, Hearthstone, and some other random stuff so I have to put other games on the HDD (DayZ, Rust, ESO, etc). But the price is still pretty steep for SSDs per GB.

If I were to upgrade something performance wise, what should I go for first?

[ebah]

I would buy SSD only (Samsung 840), because your GPU is just fine for wow imo.

[Deleted]

240-250GB Crucial M500 or Samsung 840 EVO, the rest looks fine.