I'm running SP3 on my laptop, Lan/entertainment rig, file server, and family computer with no ill effects, they are just as stable as before.
My friend installed SP3 on to his newly built computer and had sound issues (however, I think the fact that he installed Via's 4-in-1 drivers into his system on a non-Via board might have something to do with it )
The same guy introduced me into computers and now I know so much more than him
The only issue I had with SP3 (which might be an isolated case) is when I reformatted my entertainment rig and freshly installed win2k... I installed SP3 first and the computer wouldn't boot any more. It couldn't find the DTLR file (something to that effect, boot loading file). However, after I reformatted it again I updated IE to 5.5 and then installed SP3 and all was well.
Also, the install doesn't take too long if you have the actual SP3 installer instead of downloading it each time you update. (so that means it's just a one time download and you can use it whenever you want to, it's around 124MB for the entire thing if I remember correctly)
Hey there FrankBlack, have you considered putting your army of computers into use for distrubuted computing (if you haven't done so already)? I thought I had a lot of computers... but man, nice fleet!