f_osc will be at 1.8432MHz, the accuracy at that baud rate should be much better. I believe I can achieve this number with the correct code but I don't know that code...
You can't change Fosc just by changing code; you have to change the physical crystal/resonator attached to the chip.
18.432MHz would probably be a better choice, but you'd probably still need to change a lot of other Arduino code.
With the usual 16MHz resonator, 57.6kbps should be THAT far off (about 2.1%, whereas async communications will normally allow a combined error of up to 5%.) WormFood AVR BRG calculator
Now, it turns out that the Ardunino code contains a hack that causes 57600bps to be LESS accurate than it could be, in the name of backward compatibility. If the normal bitrate isn't close enough, try using:
Serial.begin(57601);