The HD4770 or HD5670 are not ideal for crossfire. It is better & cheaper to buy a single more powerful card than 2 of these relatively weak cards.
We will be able to help you better if you will tell us what will you use your PC for? Gaming or video editing or just productivity (excel, powerpoint,work) & browsing.
Another important consideration is what is the monitor you are using ( size & resolution ) example: 21.5" at 1920x1080 or 18.5" at 1366x768 or 17" at 1280x1024.
Information about your processor & motherboard specifications are very useful too ( download cpu-z) to find out these info without opening your PC case.
Based on this, we will be able to make better suggestion that is optimum for your requirement.