If you get another 6770 and it does not match your current GPU's clock speed, it won't really matter. When you connect the two in crossfire the cards will default to the speed of the slowest card. The crossfire chart at ATI/AMD also shows you can also use a 5770. And indeed, the 6770 is just a rebadged 5770(right down to it's 108 watt TDP).
I might as well mention that you can also use a 5750 or 6750. Although either would cause your 6770 to drop at least 150 MHz, to match at 700 MHz.
Just make sure whatever you get has 1 GB of memory.http://sites.amd.com/PublishingImages/Public/Graphic_Illustrations/WebBannerJPEG/AMD_CrossfireX_Chart_1618W.jpg