Asus P8Z77-V LX

Do not get a new motherboard then, instead sell your current graphics card and buy a more powerful single card.
Crossfire does not work well on low - mid range cards. Its ALWAYS better to get a single better card, so few games can actually use 2 graphics cards at the same time.
 
You should have no problems with your current board. Total bandwidth is 16X and even the most memory hogging games don't come close to the 16X bandwidth. I would try the dual radeon 7750 configuration and see what kind of FPS you get. It will be a decent gaming experience. If there are any problems it will be with the CPU possibly bottle necking the data. Give it a try first. One step at a time.