@Panga:
I used VirtualBox 3.0.10 on linux.
I found the solution on wikipedia: VirtualBox does support OpenGL (only 32-bit windows XP and Vista, linux, solaris) after having installed the "Guest Additions" (see manual, it's like a setup cd that you put in the virtual operating system)
@juan974:
I have not yet thought about it... Interesting . The XP on the Virtual Machine cannot be completely compared to a normal XP on a real computer, VirtualBox does not support all techniques that Sketchup can use, like hardware acceleration. Also I used a very stripped down Windows XP which can of course not use the full ram (in my case I allowed it only to use 192MB) because otherwise the host operating system cannot work. The minimal XP install cd can be created with nLite.
Luckily SU7.1 works now also in Wine on linux without problems, so I only need VirtualBox for placing models in Google Earth. The above test made me wonder why SU runs faster in a virtual machine than in wine...