An Atmega UART can work up to about 2MHz IIRC, but if there's a lot of data you will not be able to keep up.
If the data is in bursts you should be able to buffer it in RAM then deal with it at the end of a packet. And you have to detect the end of packet as well (I don't know how that's done with CAN) while buffering data so the code will have to be pretty tight.
Atmel do make chips with inbuilt CAN support.
Rob