No, I was correct in thats a normal h55 board, the h57 made me double check... but its a 1156 setup. So it should take any current 1156 cpu, i3/5/7, as all the 1156 cpus came out at the same time. So no back pedelling, just a double check.
The swaps quite easy, Abit involved, as you'll have to struggle with the dumb intel push pins for the heat sink, but its quite simple. I wouldnt pay some one to do it, just watch a few youtube videos.
As for which cpu, it won't be faster per core, but as theres 4, it'll be able to do more at the same speeds. Which if the program can use many cores, this makes the program run faster.