I'm hearing from others they're getting the same message you are with the same card. It seems FCPX 10.0.1 changed some things in GPU support. There had been people who were using the ATI 1900 with 10 that now doesn't work with 10.0.1.
They must have just updated this. This is list of Unsupported cards. Lists 3870 as not supported. Shows the list was updated yesterday.
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My 2600 wasn't supported from the get go so I bought an ATI 5770 from Apple online.