A sli gtx 560 ti is the same amount as only one GTX 580 but should I get the 580 and sli later on or just go for the 560 ti sli?

Intel Core i7 920 2.66Ghz

Gigabyte GA-EX58-UD5

eVGA GeForce GTX260

Corsair 6GB Kit (3x2GB), PC-12800 (1600MHz) DDR3

Western Digital 1TB, Caviar Black, SATA-II, 7200RPM, 32MB Cache

HAF 932

850 watt PSU

580 alllllll the way. Plan ahead man, and the 580 will run anything you can through at it, and well. Never settle for second best unless it's the best you can get.

The 580 is more expensive than two 560s and the 560s with SLI are more stable with better performance.

I would just go with one card, as a person who has done both I personally have had better luck with single card configurations from the amd side of the house at least.

common rule is 1 card > 2cards 

560ti SLI is faster sometimes and the single 580 other times.

I personally do not think SLI is worth the hassle.

I went with B. Personally I run 2 570's, slightly overclocked, in sli. If there is a good price drop and this card is still available, I may go 3 way.

Forgot to mention current specs-

Intel Core i7 920 2.66Ghz

Gigabyte GA-EX58-UD5

eVGA GeForce GTX260

Corsair 6GB Kit (3x2GB), PC-12800 (1600MHz) DDR3

Western Digital 1TB, Caviar Black, SATA-II, 7200RPM, 32MB Cache

HAF 932

850 watt PSU

gtx 560 sli, you get like 20% or more power than a gtx 580. It depends where you see the benchmark which generally favors two gtx 560. http://www.guru3d.com/article/geforce-gtx-560-ti-sli-review/7  
 
Even these benchmarks supports my claims http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_560_Ti_SLI/8.html 


 
Which is great in itself, but others will tell you to get the most powerful single card you can afford like the gtx 580, then sli that. Then again you need a bit more power, so this is the right combination. This means you probally don't have to upgrade for the rest of the gen. You live in austrilia so you pay more, but this card in north america is like $400 for two gtx 560 which will smoke a gtx 580 for $500 or more. 
 
 

i got a 580 myself and plan to SLI it probably next year when i upgrade my "lowly" i5. fucking amazing card

@Rhaknar: I remember you, didn't I recommend you a gtx 580 single card? Is the fan still loud? Every game works well? Any games has a problem or not fast enough?   Well for the op, i'm doing the opposite, recommending him two cards if it's cost effective.

I would go with 580. Its a beast and it should serve you very well for a while.

Personally if i had SLI support on my current motherboard i would go with the 2X 560's, they are cheaper, and the performance would be equal if not better than the single 580.

Personally I would go with the 580, first of all less hassle, second of all, it essentially future proofs your rig a lot longer than the two 560s, particularly if you plan to sli later. 
 
Also; 
 
What the fuck are you running which requires this much power!?