1) thanks for answering my question so fast.
2) I am playing at 1920x1080, So I know that there is going to be performance decrease when I will use gtx 460 768mb. So is it worth spending Approximately 90$ for the 1gb in sli? I am open for other suggestions too (maybe gtx 480, hd5870)
At 1920x1080 I'd suggest doing SLI GTX 460 1GB. The 768MB version is going to limit you at that high of a resolution. So I'd disqualify that one to be honest.
Also, an SLI GTX 460 1GB setup will perform about 20% faster than a GTX 480. And the GTX 480 outperforms an ATI 5870. So with those three options, the SLI GTX460 1GB setup is the best performing option.
If you can afford to do it, either go for SLI 460 1GB, or get a 5870/GTX 480 in some benchmarks. The GTX460 768MB isn't a great idea.
Now, on the other hand. Pricing on the GTX 470 is coming down pretty quickly. And a single GTX 470 performs pretty well @ 1920x1080 by itself with most games. However, it will run louder & hotter than a couple GTX 460's. So you have to weigh out pricing. SLI 460's is more performance than you need (which is good long term), but a GTX 470 is probably plenty for now.
Yes a 760/750 will be powerful enough for those cards depending you'll wont be missing out on any significant performance going that rout, although some games may like a small oc to the cpu but that's just because not all games are optimized very well.
I also agree going the 1gb version just due to most sli/cfx set ups run alternating frame rendering, meaning the gpu renders one frame and the other renders the next, so for higher textures and other things in games at your resolution a 1gb card would be slightly preferred.