GTX 580, HD 5870, GTX 460 SOC

I think that two 460 1gbs will perform at roughly the same level as a 580, but there are always issues with SLI. Even so, dual 460s would be a very good route if you don't want to buy a single 580. If you want a more reliable setup for a higher price, a 580.

Edit: You might want to wait for Radeon 69xx to see what comes out. The 6970 could very well beat the 580 for a cheaper price.

I always choose single card solution, it gives you less problem, less heat and less power consumption...
So i think getting GTX580 is really good plan.  

^^ I like his plan. Also going SLI will mean you have to spend more money on the PSU, MB, Cooling...

I would go for the single GTX580, allowing you to upgrade in the futute to SLI. Having said that, I think your choice of the Gigabyte SOC version of the GTX460 is a good one. That is a quiet and fast card, for a mid-range price.
http://www.guru3d.com/article/gigabyte-geforce-gtx-460-...