Site:tomshardware com amd radeon hd 7970 with 620w

Yes is good. You can overclok the video card with 620W PSU.

*Note it's NOT just the Total Wattage. The PSU must also have sufficient Amps on the +12V rail (or combined rails).

HD7970 requires at least 40Amps. It is very overclockable and a 25% overclock could consume 55Amps of current!!

I recommend a PSU with at least 650Watts and 60Amps to be safe. About $100 if you shop around.

Read my comment and keep that in mind:
http://www.tomshardware.com/forum/328808-28-power-suppl...

http://www.amd.com/us/products/desktop/graphics/7000/79...
500W (or greater) power supply with one 150W 8-pin PCI Express power connector and one 75W 6-pin PCI Express power connector recommended.

here is the rest hardware:

CPU : i5 2500 3.3 GHZ
HDD: Western Digital Caviar Black 1TB
RAM:6gb

Edit :

I found that PSU :

http://www.corsair.com/enthusiast-series-modular-tx650m...

Is this good enough?

Will be good.
If is one of these : SeaSonic M12II 620 Bronze 620W http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Antec NEO ECO 620C 620W Continuous Power http://www.newegg.com/Product/Product.aspx?Item=N82E168...
It will be fine.

YES. The HD7970 (non-overclocked) needs 40Amps. This PSU has 54Amps.

IMO, the best card available is the HD7950 OC from Saphire Tech:
http://www.legitreviews.com/article/1862/

It has a GREAT cooling solution, can be overclocked, has a dual BIOS and costs about $490 at NCIX (USA). It was UNOFICIALLY overclocked by 45% though I would not do that myself. I would overclock to 20% above stock. Keep in mind you need MORE AMPS to overclock and it's not proportional. A 25% overclock needs close to 40% more Amps so keep that in mind.

You can use the above PSU and HD7950 OC and still have a modest overclock on the graphics card. Run FURMARK to test the graphics card for stability.