The P8B75-M LX is a great motherboard, but is microATX and only has two memory slots so if you get that you would have to totally replace your memory to go from 8 to 16GB, although 8GB is really plenty. The HD9000 will work in either.
The LK is aboard I have used many times in budget based systems .... only odd issue was CPU LED being on for a GFX card failure. As much as I like the board, with 1155 being a dead socket, I'm buying only 1150 socket boards for new builds today
As for your questions "do mother boards matter ?" ..... Id say so.... kinda like cars....they both get you to work each day, but some have better performance (acceleration / braking), better efficiency (mpg), better features (AC, sound, traction control) than others. More and better BIOS features, better overclocking performance, better / more robust components, number of clots, slot spacing, number of fan headers, type of fan headers, type of LAN, sound etc all may have an effect on which you choose.