I would advice against building a new system using multiple GPUs as it is more expensive than using a single higher end card as you need,
A beefier PSU, to support two cards.
a high end CPU, otherwise the CPU can be a choke point,
A case with very good airflow to cope with the added heat.
Prepared to spend time configuring the system to run stable.
Exception would be if you already had a system with a weak card and you could find a second one for cheap, but you would possible need to upgrade the PSU.
For Crossfire, you need two cards,
for Hybrid Crossfire, (using the iGPU of the CPU) you need an AMD CPU and MB.
Comparability list. > http://support.amd.c...fire-Chart.aspx
HD 7700 single vs Crossfire, > http://www.techspot....fire/page4.html
I would sell the card and buy a higher spec new one.
Your new parts.
Using a B or H chipset cuts down the available total bandwidth and precludes any overclocking, except Turbo Boost.
If you provide us a little more info, we could configure a build for you to consider.
Do you want to reuse any existing parts? please list brand and model, including PSU.
In which country would you purchase parts?
Your current OS.?
What would be your budget?
Note, if you OS is an OEM version, it is tied to your existing MB and is not transferable to a new one, .