I wouldn't do something so foolish as play around with my rig "hot." Shut down. Think about placement before you fire the rig up. Others might swap their fans, but do you really want to take the risk of burning up all or part of your rig because you didnt want to shut down each time. Think about it.
You have a 1.2 GHz Athlon, and ASUS mobo, 1/2 gig crucial ddr, 64 meg gf2 ultra, 80 gig WD, SB Live!, and a Plexwriter that you are willing to risk shorting out?
Even if you are just playing with the fans, are they NOT connected to the same powersupply that powers all of you other equipment?
I wouldn't play with that kinda fire.