9-speed rear derailers, front derailers, chain rings, chains all work fine with 8 speed, as I'm running all of these on an 8-speed. Only trick is shifters and cassettes. Currently I'm running Alivio shifters, and I think they work fine. In fact I can barely tell a difference between my friends bike with LX shifters. I pull the triggers, the bike shifts what more can I ask for?
For cassettes the SRAM 5.0 is ace, and for $30 bucks CDN an absolute steel!