What's the best graphics card for my system?

You can get a 7870 for that price :

http://www.newegg.com/Product/Product.aspx?Item=N82E168... (although not recommended cause of the black screen problems).

Or a 660 gtx (recommended) :

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

The psu though is from an unknown brand, meaning it's not a reliable one.

Thanks for your answer: by the way...I'll be using 1680x1050 resolution at least for the time being, in the future I may get a fullHd monitor but I don't intend to go any further.

The 7870 cost about the SAME, and it has better graphics.

The Gigabyte 7870 that I linked below is 230 u$S, but it comes with Triple Windforce! And it would make the card never go higher than 60°C Just Amazing! Fits perfect with an i5 and with your resolution preferences  
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

btw, CPU and graphics card are totally different, having one from AMD and one form intel will never make the difference keep that in mind.

A 7850 can be over clocked very easily, basically making it a stock 7870, and for less money.

My 7850 cost me $189 from NewEgg. http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Also, I suggest you first get a different PSU as someone above said. You risk A LOT trying to game with a powerful video card and a cheapo power supply. If the investment you have made in your computer means ANYTHING to you, get a decent PSU.

I can not stress that enough.

Go for Corsair, SeaSonic, or Antec. There's a few other good ones, but those stand out in terms of quality and reliability in my opinion. Good luck!

I agree with MaXimus421
I didn't noticed the cheap PSU, with the 189$ of the 7850, you can also buy a decent PSU with the difference of the card I suggested
Just like this Antec 450W
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

That way for your budget you get a nice card and a great PSU too.

Why do you say my PSU is bad?, just because it's not a known brand?...I mean, a manufacturer can make good components even if it's small. I purchased it from a PC repair shop and I think the owner is a reliable technician, I told him I wanted a good PSU to add a good graphic card and he sold me this one.
Here're some specs in the box for this PSU:
ENERGY PS700SX
12cm sleeve fan
20+4 pin connector
TC Temperature Control
4xSATA connectors
4xIDE connectors
1xPCI 6pin connector
P4+P4 Passive PFC

A psu with 700w that comes with a single 6-pin pcie connector and only 4 sata and without any certification of efficiency is not a good one, and it's missing one important detail the +12v rail(s) with the respective amperage.
That being said, it doesn't mean it can power a 7850 or a 660 gtx, those cards are lower power demanding for its performance.
If you want to keep the psu, i advice you to go with a 660 gtx or the 7850.
If you can find a 660 gtx bellow 215$ get it. it's better than the 7850.

The zotac 660 gtx on newegg(currently out of stock) or the asus 660 gtx in ncix :

http://us.ncix.com/products/?sku=76367&vpn=GTX660-DC2O-...

The lowest priced GTX 660 I've found is PNY GeForce GTX 660 XLR8 Edition OC - 2 GB GDDR5 - PCI-Express 3.0
http://www.pixmania.com/es/es/12821060/art/pny/geforce-...
...there's another MSI GeForce GTX 660 OC - 2 GB GDDR5 - PCI Express 3.0 (N660-2GD5/OC) for 12 more dollars.

I'm in Spain

Thanks for your answers

djangoringo: I've ordered the PNY GeForce GTX 660 XLR8 Edition OC - 2 GB

Thanks for your advice