The motherboard you chose should be fine if you wanted to add a 3rd and 4th card later. The PCIe 1x to 16x converter is just that, the cable plugs into your 1x slot, then on the other end, it has a slot for you to plug your 16x card into.
Ok thank you
just to be sure somebody else could confirm ? and rummaging through my stuff I've found an old pc (celeron 2,x gHz, HDD 80GB, 1,25 DDR), am I use the celeron and a motherboard whose accept it on our rig ?