Could you tell me how to improve performance without big quality loss beside of making chnages in video options ....?

Perhaps exist plugins that could help increasing performance and fps?

I just posted in your thread in the Mods forum.

First you should post a dxdiag, to see what background processes you can eliminate.
Post a dxdiag.
XP: Go to Start->run, type “dxdiag” and hit enter. Click the Save All Information button near the bottom right. This will save a Dxdiag.txt in C:\ by default. Then open the dxdiag.txt and copy/paste the information into your post.

VISTA/Windows 7: Go to Start->Search, type “dxdiag” and hit enter, click the Save All Information button near the bottom right. This will save a Dxdiag.txt in C:\ by default. Then open the dxdiag.txt and copy/paste the information into your post.

One thread is enough.