FreeBSD 8.2 released
freebsd.org
freebsd.org
Here is one that I have heard of: http://www.ixsystems.com/bsdsupport
I'm sorry I don't have any experience with them, but you might be able to try them out.
As an X desktop you can run gnome, kde, xfce, xfwm, or really anything. Some components are Linux only (Xfce was recently bit by this) and thus may not work as expected or poorly if there is no abstraction layer that allows for us the use of devd(8) for example.
FreeBSD is my favourite OS for servers, it has good hardware support there where it matters most, and best of all is extremely stable. If Linux has met your needs so far, or even Windows, then stick with it. You won't find anything new and exciting and may even find it frustrating that certain things don't work as expected due to differences in API's that are available.
If you want a distribution of FreeBSD that is pretty well geared towards desktops, may I suggest taking a look at PC-BSD. They generally are not too far behind the official release of FreeBSD with their FreeBSD version, and it is an KDE environment that is easy to install.
If you're ready for the real thing - go for straight up FreeBSD. They are extremely organized, you can go to one site for all of your questions, everything is well documented. They have pre-compiled third party binaries available known as packages in addition to all of the source which you can compile yourself. There is also a 'Linux compatibility mode' so that you can run Linux binaries.
I personally prefer FreeBSD over Linux.
It seems to have good support for KDE though---http://freebsd.kde.org/
ZFS v15 really helps with the overzealous memory allocation that earlier versions were plagued with on FreeBSD. Upgrading to 8.2 gave me a 20% - 30% R/W performance increase out of the box without any sort of tuning or optimization.