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Graphic card or another HDD, maybe SSD if you can aford it. At least is what i would buy.

I'm not a "pro" but for what I've read, 2gb for graphic cards seems to be low nowadays. The SSD would make your computer runs faster, specially if you install your SO or some games for what I been told.

Wait for more accurate responses.

GPU + HDD with more space than just 500Gb, you don't need to replace the HDD, just get an additional one, unless you feel comfortable with 500Gb,, or an SSD for faster working OS.

I'd upgrade the GPU, then add an SSD, then possibly a bigger HDD if needed - in that order. I'd also be sure that PSU isn't some garbage no-name brand - would you really trust a $50 PSU with $1000 worth of components? ;)

2Gb is "too low" depending on the resolution, the games played and the general wishes of the player (are lower than maximum settings acceptable etc.)

I'd upgrade the graphics card and buy an ssd. And honestly, I'd put the SSD first, since HDDs just cripple PCs. The 660 isn't that bad for the moment, should be tolerable for a few months - new graphics cards are still fresh and the price is set to be lowered over the next few months.

Don't worry about upgrading RAM until it's full someday, in some application. CPU is fast enough for the next 3 - 5 years.

Depending on the exact model of the PSU I'd upgrade that, too, before installing a new GPU. Sucky power supplies are quite hazardous to your PC - in terms of efficiency and "frying" your parts.

GPU definitely. Your CPU is good, RAM is good, PSU is good, OS is good, Everything is good but a better GPU can work wonders. Depending on your budget I would get one of the new AMD 300 series or a 980. (or more if you're gonna spend more than you got the computer for)

An SSD would also be nice but a GPU would be a better upgrade.

You don't need an upgrade, seriously. That gpu is fine. If you really want to buy an upgrade, I would wait a few months at least, to see how your rig handles the next games.

How do you use it without a motherboard?

If you want to see a really good improvement, buy an SSD.

What kind of psu do you have?