Finally I found out what I really need. Of course Atmel refers to the datasheets but the datasheets are not very clear. The really useful information resides here:
AVR532: Migration from ATmega48/88/168 to ATmega48A/88A/168Ahttp://www.atmel.com/dyn/resources/prod_documents/doc8280.pdf
AVR505: Migration between ATmega16/32 and ATmega164P/324P/644Phttp://www.atmel.com/dyn/resources/prod_documents/doc8001.pdf
AVR042: AVR Hardware Design Considerationshttp://www.atmel.com/dyn/resources/prod_documents/doc2521.pdf
It just never occured to me that the migration notes could hold such useful information while the datasheet does not. Now it is trivial to select the proper crystal and the proper load balancers. Do not ask what I will use. If you need the information read these 3 notes they contain everything that is needed.