GTX 560 SLI vs GTX 580 1.5GB

You should clear that the the gtx 560 you are saying is the ti or non-ti version.
Anyways if you sli the non ti version (which is the weaker of the two), it will give you more than enough performance for your resolution. So you can save some cash by going for the sli setup. The gtx 580 is more expensive than 2 gtx 560. You can use the saved money for other uses like better cpu cooler, better psu etc.
If you do not feel comfortable with sli, then a gtx 570 is a great option. It is cheaper than 2 gtx 560 and can give great performance for your resolution.
If you want to go with AMD card (which will not fulfill your desire for physx) you can go for radeon 6950 2gb (if you are low on budget) or radeon 6970 or either crossfire radeon 6870.
But on your resolution , you should definitely grab a gtx 570 and save some cash.

Neither Crysis nor BF3 use GPU accelerated Physx. There have been only a few games over the last couple of years that use it in a worthwhile manner, notably Mafia 2 and Batman: AA. I highly recommend you don't use it as a deciding factor about what video card you purchase. It just isn't very important.
If you barely have enough money to build a computer why are you looking at spending $400-500 on a video card/video cards considering you will only be at 1080p?
I would get a single GTX 560 Ti with good cooling and overclock it up to around 1ghz. It should do very well at 1080p. You can always add another in the future if you feel like it.

Ya, i barely have money to upgrade. The rest components are around 600€. I want something that will last a while, i don't want to be upgrading again at Christmas. That's why i am willing to spend that much money on a graphics card. Finally, i think i'll buy the GTX 580 1.5gb.. And add another one in the future if games become more demanding. What do you think?

The GTX 580 is not a good purchase in terms of value for the money. Graphics cards are a very fast moving technology and buying an ultra high end card like the GTX 580 currently will always come at a steep premium and never make sense on a budget. Buy something that is sanely priced and well suited for your needs like I suggested and upgrade as necessary. Two GTX 560 Tis are notably faster than a single GTX 580. I still suggest you just get one for now and overclock it. You will likely find you don't even feel the need for a second card for a long while and by then there may be cards on the market that make more sense than buying a second.
Have you actually looked at benchmarks for these cards at your resolution?

580 breaks most games in half single handedly. Its a single gpu. Not any single gpu faster out there. Of course its a good purchase. Have you ever played 3d?

Lots of questions lol. Ya, i have looked at the benchmarks jyjjy, and i'm satisfied.
gnomio, yes i've played 3D and i totally can't stand it. For me at least, 3D sucks.
But, recently some new sources came to my attention, and i might have some more money to burn- a small boost. I probably get the gtx 590, after all. And i don't wanna hear <<OH IT'S INSANE OVERKILL!>>, yea, i know it is, but i will be really happy playing crysis 2 on directX 11, with high res textures pack enabled, and all settings on max, and still get 50+fps.
And, from an aspect, no it isn't overkill, in 6 years from now, gtx 590 will be like the today 9500GT or something lol.

3d question was for one above me.
What's your native resolution? What's the rest of your setup?
Nevermind. If you want to go for the 590 add another monitor

As said multiple times above, res is 1920x1080.

Rest is:
Intel Core i5 2500k
GIGABYTE GA-Z68P-DS3 mobo
Corsair Vengeance 8gb ddr3 1600mhz
CORSAIR Professional Series Gold AX850 80 Plus Gold Certified 850w Power Supply
Antec KUHLER H2O 920 Liquid Cooling System (CPU Cooling)

The case is Cooler Master HAF 932. I hear it has amazing overflow. Also it's a full tower case, i'll need it.

Also a single 1.4TB HDD

It might be a slight overkill for my single monitor, but it will actually make games cry.

Except if one is on a more reasonable budget it makes less sense, as with most computer hardware to buy the very best; the frames per dollar always steeply drops around the "fastest out there".

Yea, i am getting the gtx 590, now there's another problem. The mobo is 5 pounds! I can't have it sent from US to my country!

@paliovouna, watch your language!  

I think I have around 12-13 PhysX games. PhysX might not be "the" deciding on video cards, but it definitely bears some consideration.

I'd add Cryostasis, Mirror's Edge, and Metro 2033 to the list of games I liked using PhysX with. Also, when you buy a new card you usually keep it at least a year, so you don't know what will launch during the year that uses it.

Agreed on the 560Ti being all that's needed for 1080P, I'm using two 2GB 560Tis for 57X10 these days, they're very good "bang for buck" cards.

The GTX 580 STILL is simply a poor choice for the money. As I said earlier in answer to your initial question two GTX 560 Tis are faster for the same money, especially if you OC, but if you want to do it right I would recommend crossfiring HD6950 2GBs for a little more. They would be 35-40% faster than a GTX 580 on average and the extra memory can really help get the most out of that kind of processing power, especially if you switch to 3 monitor gaming/Eyefinity in the future(a good idea.)

judging from your other thread it feels like you just wanna throw your money at the best card regardless of how good or if its the smartest buy, which you have every right to and i will not stop you. i just want to remind you that no one wins in a pissing contest. also people have recommend better setups for cheaper so if you wont listen why ask questions?

Couldn't agree more, even though I did spec one out for a clients rig that I've just built.