If I recall correctly, an LX derailleur will work fine with your 8 speed setup. The difference is in the amount of cable pulled by the shifter for 7 vs 8 vs 9 speed. Bear in mind that derailleurs take a lot of beating so you will end up with a scratched up LX soon enough. Have you tried sachs chains with the powerlink - I've had really good results.
Laters.