From the PC you certainly need a level shifter, the MAX232 or MAX3232 come to mind. RS232 levels from a PC are typically +12V to -12V so far too high for any PIC but note the levels are inverted too.
You might be able to use a direct connection between the PICs, although not ideal, the 3.3V signal should be recognized as logic levels by the 5V device and to drop the voltage in the other direction you can use a resitive divider, for example 1K in series and 1.5K to ground on the 3.3V side. If long distances are involved, certainly look at using line drivers and receivers but for short distances it will almost certainly work without special interfaces.
Brian.