better performance

No you can only crossfire two of the same cards IE two hd7770's or two hd 7970's
I would save your money and get a high end card like a 270 or 280 and be done

wagex said:

no budget? get a 290x / 780TI and call it a day.

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What he said.

I would save up and buy at least a R9 270x for BF4 but since you have no budget and can save for anything I would buy a GTX 780ti.

Listen to what the people here are trying to tell you and you will get good advice. In order to crossfire you need a similar gpu architecture with the same size memory buffer.

Ie. 7950/7970 7850/7870 6850/6870 just a couple of examples.

G14NN1S said:

if i buy the Gigabyte Radeon HD7790 2GB OC will i be able to have both of my cards (Gigabyte Radeon HD7790 2GB OC and asus radeon hd 6670 1gb)

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And none of the cards you're listing will work with the current one you have, as has been stated several times so far.

G14NN1S said:

if i buy the Gigabyte Radeon HD7790 2GB OC will i be able to have both of my cards (Gigabyte Radeon HD7790 2GB OC and asus radeon hd 6670 1gb)

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This was explained already. No.