Hi Skorper,
You can find out on page 23 or so of the 16F84a data sheet. The chart shows the lowest two bits are for the OSC configuration. As it is, your hex value converts to bin 11 1111 1111 0000. The least significant bits are on the right, 00. This selects LP osc.
Here:
So to select the XT OSC, you need to change that to bin 11 1111 1111 0001, which is 0x3FF1.
To select the HS OSC you need to change it to bin 11 1111 1111 0010 which is 0x3FF2.
You can use the windowz calculator in Scientific mode to do conversions like this quickly.
I should point out that 0x3FF2 is the same as H'3FF2'.
I hope this is clear to you, but if you have further questions, just ask.