How strong should my PSU should be?

http://extreme.outervision.com/psucalculatorlite.jsp

Around 550 watts plus for a single 560ti or at least 750 watts for a SLI config

A quality 500-550watts psu to run you with just one card
A quality 750-850watts psu to run you dual card.
Since you are getting the 2500k i take it you are planning on OC. In which case i strongly advice investing on 850watt (plus,bronze,silver,gold from antec/enermax/corsair)
If you don't plan on OC drop the 2500k go to normal 2500 or even 2400.
I don't see the point of your dual hdd configuration. Either single drive on a fast hdd or jump on the z68 wagon, get a small 30gb caching ssd and watch your system fly.
That being said, there are a lot of budget z68 mobos out there. Personally i find asrock extreme3 gen3 to be the king of the budget hill.
Your chosen gfx card is nice but performance/price wise i would pick another. Considering it gets outperformed by other cards at it's price range and this seems to be a budget build, perhaps you should reconsider.

my bad, i read again you are planning on OC. Scrap the part where i rant about locked cpus.
To cap it up then... For your prospects of OC dual card beast -> Quality 850watt (considering you might go wild on the cards OC too) and don't go cheap. If a cheap PSU goes boom along with the money you paid for it, your precious rig might go as well.
Consider going z68. An ssd, even if it is just a caching ssd will net you amazing speed bursts.
In my opinion what counts is what is on the screen and not what is in the pc case. Get a less fancy but more powerful card for the remaining money you got.
Don't skip the ssd!

First of all thanks for all your suggestions m8 helped a lot 

Secondly about the 2 HD its just that i already have the 250GB one so since i think it isnt big enough for my needs i am planning on buying one bigger and use them both.

Now about the PSU part i was thinking on going either of the 2:

Corsair GS 800 or Corsair TX 750 v2

but i am now looking at the Corsair TX 850 and trying to decide is the its worth the little extra money.

Are you going for dual card setup then? Don't narrow it down to Corsair. Enermax/OCZ/Antec/Lepa/Seasonic all offer quality PSUs.
Find out what you want to put in the rig and how bad you want to OC it.
You can also run it through this page to be 100% sure
http://extreme.outervision.com/psucalculatorlite.jsp