The first one cost $400 and the second $200 each one.
My computer has a intel ivy bridge i7 processor and 8gb of ram.
I want to play games and 3d renderings.
What combination is better?
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It is usually better to have one very fast card rather than two half speed cards.
Reasons for using one fast card:
Reason for using two cards:
Having said all that, the 7870 is not half the speed of the 7970. It is about 60% of the speed. Nor will you gain double speed with SL/Crossfire. The speed gain is usually around 80% to 90%, depending on the application. (Read, some games scale better with more cards. Some do it poorly. And as stated some do not do that at all).
Added a card to make comparisons a bit easier.
The 7970 has a PassMark score of 5093; the 7870 has a 3442. As such, the 2x7870 could theoretically exceed the performance of the 7970. However, it wouldn't do so by a wide margin, so I think I'd stick with the stability and upgradeability benefits of the single-card configuration.
But wait! You say you do 3D rendering; in Blender, per chance? If so, you should probably know that the Cycles engine doesn't support AMD GPUs as of yet, and it doesn't look like it's going to for a while. This is the primary reason why I regret buying an AMD card.