if you have crossfire connectors, a strong psu, case, mobo, cpu - you can cfx 5830 with 5970. 5830 can be cfx with 5970 because it has two 58xx series (forgot which exact ones
) gpus. the whole combo would run at 5830 speed.
check the compatibility list here:
http://sites.amd.com/us/game/technology/Pages/crossfirex.aspx
http://sites.amd.com/PublishingImages/Public/Graphic_Illustrations/WebBannerJPEG/AMD_CrossfireX_Chart_1618W.jpg
i see that the 5970+5830 is not marked 'recommended', however it is possible.
web search for that cfx combo in case it loses too much performance with 5830, it won't be worth the combo.