A good gpu is better than an expensive cpu for gaming ?

For gaming, the graphics capability trumps the cpu capability just so long as the cpu is adequate.

Permit me a rant on Dual graphics cards vs. a good single card.

a) How good do you really need to be?
A single GTX560 or 6870 can give you great performance at 1920 x 1200 in most games.

A single GTX560ti or 6950 will give you excellent performance at 1920 x 1200 in most games.
Even 2560 x 1600 will be good with lowered detail.
A single 7970 or GTX690 is about as good as it gets.

Only if you are looking at triple monitor gaming, then sli/cf will be needed.
Even that is now changing with triple monitor support on top end cards.

b) The costs for a single card are lower.
You require a less expensive motherboard; no need for sli/cf or multiple pci-e slots.
Even a ITX motherboard will do.

Your psu costs are less.
A GTX560ti needs a 450w psu, even a GTX580 only needs a 600w psu.
When you add another card to the mix, plan on adding 150-200w to your psu requirements.
A single more modern 28nm card like a 7970 or GTX680 needs only 550W.

Case cooling becomes more of an issue with dual cards.
That means a more expensive case with more and stronger fans.
You will also look at more noise.

c) Dual cards do not always render their half of the display in sync, causing microstuttering. It is an annoying effect.
The benefit of higher benchmark fps can be offset, particularly with lower tier cards.
Read this: http://www.tomshardware.com/reviews/radeon-geforce-stut...

d) dual card support is dependent on the driver. Not all games can benefit from dual cards.

e) cf/sli up front reduces your option to get another card for an upgrade. Not that I suggest you plan for that.
It will often be the case that replacing your current card with a newer gen card will offer a better upgrade path.

On the cpu side, most games can only make use of 2 or 3 cores. 6 and 8 core cpu's are good for multithreaded apps, but not for gaming. Gaming depends more on a few fast cores. Today, AMD is not a good option in the >$200 price range. Intel sandy bridge chips like the 2500K are as good as it gets. The sandy bridge cores are much more effective on a clock to clock basis. In April, the ivy bridge follow on cpu's will give you about 10% better price performance. Even a good sandy bridge duo like the $150 2130 or $100 G860 can be good gaming chips when combined with a good graphics card.