If you use a 40MHz device (10Mhz Xtal with PLL x 4) then your instruction cycle is 100ns. So:

In assembly:

bsf portb.1 ;set port B, bit 1 high
bcf portb.1 ;set port B, bit 1 low

This consumes 200ns which is 5Mhz.

But as you can see there is no time to do anything else...!

So leave the RF generation to some other chips (like PLL) and control these chips with your PIC.

Ioannis