Codec support is getting better due to the ffmpeg people. Most things are well supported, except anything protected by DRM. VLC has quite usable wmv support, for instance.
Problems are things like embedded flash or other sorts of embedded video in webpages is very ackward in Linux, but that sort of thing is a problem for 32bit also. Just add bad flash support for 64bit linux.
If your using youtube there is a handy script called youtube-dl that you can use to download the Flash Video ( *.flv files ) and you can play them with mplayer or vlc if it's new enough.
For running the odd 32bit program there are 2 ways.. There are usually 32bit compatability libraries that will run most stuff, like games.
Otherwise you can setup a chroot environment with all 32bit software and libraries and use that environment to run 32bit programs. This is the more reliable way. Doing this you can setup a 32bit firefox install and play flash that way.
Keep in mind that I am much more familar with Debian then with Kubuntu, but they are mostly similar as far as this sort of stuff is concerned. This manual has directions on how to setup a schroot enviroment so that it integrates nicely with your existing system:
<a target=_blank class=ftalternatingbarlinklarge href="https://alioth.debian.org/docman/view.php/30192/21/debian-amd64-howto.html#id292205">https://alioth.debian.org/docman/vie......1/debian-amd64-howto.html#id292205</a>
But to be truthfull unless your trying to do something like video editing, or have over 3gigs of RAM, or whatnot then it's probably easier to just run 32bit.