AVR USB to Serial Converter; Quick Question
Yes, the various atmega parts are likely to be compatible for this particular application. V-USB does not use many internal resources.
V-USB supports both 16MHz and 20MHz crystals. You will have to modify the circuit.
Are you going to use this circuit with AVR? Because you AVR circuit produces TTL-level serial data (0/5v), while "The Serial Link" circuit expects RS232-level serial data (-12/+12V). The serial link circuit uses all the zeners and resistors to convert RS232-level to a TTL-level in a special way. 


So the two schematics you have posted are not compatible, unless you add another level shifter / converter (MAX232) so you can go TTL->RS232->TTL, which seems silly to me.