1.) If i enable "experimental" 3D support when installing Guest additions for Guest (Win XP) OS. What do i get with that. Better OpenGL support?
You get most of the features of real 3D card + better performance.
2.) Does "experimental" 3D support require that on Settings tab for XP Virtual Machine i enable 2D and (or) 3D support. Or is that entirely different thing?
3D support only is needed.
3.) Should i have graphic card that supports virtualization to enable 2D and 3D support under VM settings. Or it will work with any graphic card.
No video card today support virtualization in hardware. 3D virtualization is done in software, so it will work with any graphic card, that supports OpenGL.
4.) Is it normal if i enable 2D and (or) 3D support that Win XP Guest OS is more unstable than when i disable it.
It may decrease stability and security in certain cases, but in general it should work.
5.) What is the "ultimate" graphic card to use with Virtual Box
huh? The best one. ATI Radeon HD 5970.

^^
Really, any OpenGL-compliant 3D card should work fine. (all modern cards available today on market)
6.) Does it matter what kind of graphic drivers are installed on guest OS. (Closed source or open source)
You must install *only* virtualbox drivers (aka Guest Additions) -- your ATI/NVIDIA drivers must NOT be installed in guest OS.
7.) When is new version of VirtualBox scheduled to be released and does it have improvements in 3D support.
Release when it is ready, and yes, it will have better 3D support. There is a chance to see it this year.
8.) Is Ubuntu 10.04 or 10.10 better to use with VirtualBox 3.2.8 or are they much alike
I think the same -- VirtualBox generally performs the same across all major supported Host OSes -- with preference for 64-bit host OS.
9.) Are there any unofficial "hacks" for better OpenGL support in VB.
Not that I know of.
One more thing:
You can select very few Video Memory for your Guest OS -- like 8 MB - this is only used for 2D.
The slider does not allow you to choose more than 128 MB. Fear not.
For 3D acceleration, VirtualBox *automatically* uses the video RAM available on the host video card.
10.)

Happy VBoxing.
-Technologov