m18x R1 upgrade to 780m or 880m?

Hi,
I would like to change my old 560m 1.5 sli to 780m or 880m. I have read that the 880m could give more problems than the 780m. Is that so? What is the difference in performance between the two vga? is so much?
I've already seen, opening the laptop, that the first vga has an heatsink of 100 watt. is the same for the new vga? not so, right?
and What about x bracket?

last thing: there is still someone buying my old vga?

Thanks for support and sorry for my English