What should I upgrade?

I am purchasing a pre-built computer. It has an i5-3570k, 8gb of ddr3 ram @ 1333 mhz, 1 TB 7200 RPM SATA 3 HD, a 7750 1GB graphics card, and a 500W PSU. It only has 1 PCI-e x16 slot and it's only a B75 chipset. The motherboard will be upgraded eventually, but right now I do not need the extra PCI-e X16 slots or much OC capability. I have about $250-300 to throw at it. I am wondering what I should upgrade? Should I put in a higher quality graphics card? Or should I change out the ram to 16gb @ 1600 mhz and add a 120gb ssd to run my os and my more used programs? I mainly use it for playing Warcraft, Mass Effect games, and occaisionally some Skyrim. I would like it to be able to play Warcraft on high or ultra settings and to run the other games on a mid-high setting. What should I do? Do any of you have a better idea?

Don't purchase that computer; you, most likely, could easily build it for cheaper. I'm not sure how much you were going to spend on it, but this would be a great way to go.

Building a computer is always a good thing.  If I were you I would do that instead.  Getting a better mobo would be good but more important then that is making sure you have a solid quality PSU (a lot of prebuilt pc's cut the cost there because no body cares about the psu)

The graphics card should probably be fine but throwing a 7870GE in there would be pretty awesome. And should do more then you want.

The SSD will increase speed of the OS loading but really isn't critical... at all... If it means you have to cut costs in other areas then don't go for it.

All in all that system would be a functional system but if you're looking at buying a prebuilt pc then upgrading the mobo out of the box then you might as well just build one from scratch... It'll save you a few pennies and you will be able to make sure what you are buying is quality.