Problem solved by changing video card from the HD2600 to Nvidia 8600GTS.
Thanks for all of the replies.

Greets from Spain

First of all, thank very much for all of the answers and advices. I have two videos and a "sku" file uploaded if anyone can see if the composition is too complex:
Videos are:
http://www.youtube.com/watch?v=cYFyHbNtHls
http://www.youtube.com/watch?v=6bsdNHqQ06Y
(sorry for the quality, but you can see clearly that the lines takes too long to show up after mouse clicking).

The "sku" file is uploaded at:
http://rapidshare.com/files/99299925/sketchuptest.rar

I'm really happy for all of your help.
Greets.

(first of all sorry for my bad english)
Hello, I have a very big problem with a new computer and Sketchup 6;
it's slower than the old PC (Pentium 4 2.4GHz).
With complex drawings the Quad 6600 takes more time to complete than
old P4. I'm really in a trouble because it is intended for work
(design gardens, etc.) and the PC supplier tells me that they don't
support software and the hardware is 100% trouble free.
I've checked the OpelGL hardware acceleration and it's "on". The
version of Sketchup is 6.0.1099, and the hardware is:

MB GA-P31-DS3L GIGABYTE
CORE 2 QUAD Q6600 BOX
3 X DDR II 1 GB MB 800 Mhz. TRANSCEND
HD 320 GB SATA MAXTOR
VGA ATI HD2600 PRO 512 MB PCI-E GIGABYTE

Any help will be very appreciated.
Thank you very much.