A Radeon 7870 would be an upgrade over two GTX 260s and above the 7870, the next best purchase would probably be the GTX 670 if you don't want to do extreme overclocking.
Triple channel memory? Do you have an X58 motherboard? Otherwise, triple channel memory would be a poor purchase. Just in case you do have an X58 motherboard, here are two kits that I'd recommend choosing from:
If the motherboard supports 8GB modules (it would need to support up to 48GB or more):http://www.newegg.com/Product/Product.aspx?Item=N82E16820226262
If your motherboard supports only up to 24GB:http://www.newegg.com/Product/Product.aspx?Item=N82E16820231357
If you have an X58 motherboard, then you'd have to have a decent CPU, but if you don't, then both of these RAM kits aren't for you and you might not have a computer that can even handle such high end graphics cards. How old is your computer and what components does it have?