You will be able to SLI or Xfire all you want with that board. Both cards perform very well and trade blows left and right. The 6870's will scale better in xfire, but any "problems" you might have read about will present themselves in both configurations, so really just do this:
1. put both names up on a wall.
2. put on blind fold.
3. twirl around alot.
4. throw dart at names.
5. go with card it lands on
Asus is an awesome card maker so good choice there. But seriously, there is VERY VERY little difference in performance between these two in single cards, while the 6870's will perform slightly better than the 560's when using two cards at once.
The PCIe is something you don't need to worry about. Seriously, you are all set there with this mobo and any of those cards.
Other than that, I would suggest buying from Newegg if you can, cause you can get a free game with the purchase of the 6870
Hey Phishy thanks for the great reply
The only problem which HD 6870 has that I have read is that it is quite bigger than the GTX 560 (almost 2" in length and 1" in height) and that's why the issue in Xfiring if the PCI slots are placed close to each other.
performance wise wherever I have seen 560 Ti > HD 6870 >> 560
Yoges.. Unless you have a terribly small computer case, you will not run into any problems running Xfire with the 6870's.
If you take a look at the picture on the mobo that you are getting, the blue slots are the PCIe slots. They are two spaces apart which is GREAT for running Xfire. This means that the top card will take up the blue slot and then one slot underneath it, then there will be a space (the second little PCI slot) and then you will install the next card there. This space is plenty to keep your cards cool and running great. Alot of motherboard don't even have this extra space but still are able to run Xfire/SLI just fine. So the length and the height honestly have nothing to do with it: its how thick they are and how many slots they take up.
The biggest troubles associated with Xfire/SLI is that first, you need to have a power supply that will be able to handle the two cards (VERY IMPORTANT). It needs to have enough wattage and be of a good quality brand. Secondly is heat. The top card is going to get alot hotter than the bottom card. To give you an example, I am running two 560ti's in SLI right now. The bottom card is at 27C and the top card is at 34C. Under load, it can be more than a 15C difference in temps. So you need a well ventilated case to make sure they dont over heat.
Aside from that, you should be golden. and yes, 560ti/6950 > 560/6870. The two cards you are looking at are roughly the same in performance, you seriously can't go wrong with either one.
I would suggest you go with NVidia GTX 560 Ti simply because they have better driver support and they also have PhysX (though not many games use it and I havent seen the eye candy it is supposed to bring)
the 560ti is a very very good card. If you can afford it, it does offer better bang/dollar and will last you long, especially if you overclock it. most of them can be overclocked to 1gz, which is crazy good. But, once again, only if you can afford it and if you have a powerful enough PSU. btw, what is your psu?
The motherboard he mentioned does in fact support SLI. As far as the sata connections go, just get L-shaped connectors and you will be fine. Otherwise, go with your mobo, you won't see much of a difference.
560 is a faster card than a 6870 and when overclocked it is quite a bit faster.
Here is a review it shows a 460 OC'd to 850 beating the 6870. A 560 is just the same thing only with additional overclocking room.