Erm, I think that you'll find you may have problems trying to do that if you are definitely using mandrake 8.1 ???
I'd think it would be easier getting a download of the latest version i.e. mandrake 10 community - the only downside is that it's 3 cd's and they're 650 megs a piece.
If not, then have a look at the mandrake site, you'd need to get all the kde packages i.e kdebase, kdemultimedia, etc etc and yes there's lots of them, they come to about 170 megs or so, and then once you've got the lot, you'll have to install them individually.
The other way, would be to go here
and follow the instructions to update your urpmi sources. That way, it will then list all the updates that are available for you system - there's an additional urpmi source at the mandrake site that will give you the info for a source update that will include kde 3.2 - again, I can't say if it's gonna work or not. I think the "easy urpmi" link is going to be your best bet - it might not give you kde 3.2, but it should give you the stuff to beable to get 3.1.5 (if that's new enough for you)
hope that helps a little