why is ubuntu 32 bit version recommended over 64 bit version?

Same as above but that most 64 bit Linux applications are simply re-compiled 32 bit ones with no real optimisation for 64bit operation.

I did try KDE in 64bit and it did feel a little faster. The lag seemed to go from that slow stock KDE menu and generally felt a little faster, but did not really notice much else. I've not done enough testing to support stability because I used KDE and that is prone to instability so the rest of this is about RAM.

Unlike in windows, Linux can support 4GB ram in 32 bit and you can download the Linux server kernel for 32 bit to enable up to 64GB ram.

PAE is installed when you install Linux with 4GB or more ram installed into the computer and this allows 4GB + ram in 32bit without installing the Linux Server kernel. I don't think it's installed if you start with 2GB ram and switch to 8GB later. Then you have to manually install it.

Most people install the windows 64 bit so they can see and use all their ram and this is why you might think you should install 64bit Linux. Same applications running in linux 64 bit will generally use more ram anyway so unless you know the apps you are running are native 64bit then there is no real reason to use the 64bit linux for desktop use.

Flash support is terrible in 64bit editions of Linux. Unless your lucky and still have Adobe's dev build of 64bit Flash. Also most people don't use their Desktops for high performance computing.

edit: Adobe has a new preview available code name 'Square.'

With the 32 bit one you get a lot less problems and can install a whole bunch more apps.

I believe the problem is with hardware, not software...

I've seen a few machines just die from a 64 Bit Installation, true story!