OK, first the HI/LO thing for Futaba. The RX are tuned to optimize in the center of the bands specified. Therefore if you cross bands HI to LO or Vice versa there will be a slight loss of performance (read range/sensitivity). The LATEST Futaba FM RX on the market
R168DF
http://www2.towerhobbies.com/cgi-bin...p?&I=LXLSX4&P=
are NOT restricted to HI/LO separation and can use any appropriate crystal.
OK, now single/dual conversion, these recievers end up operating on the same frequencies of course but the crystals actually oscillate on different freqs because there is another step of intermodulation(mixing) in a dual conversion RX. I do not know if all brands of RX use the same IM freqs or not. I always recommend buying the correct crystal from the mfg of the RX. If you can't afford a $13 XTAL after buying a $600+ plane, then park it for a week or two.