Hi! Im working as a geologist, and im doing Environmental Impact Assessment (EIA) and Screening. I need to model landscape and geology of the ground. Can i do it with microstation or autocad…or are there any specific programs for that kind of modelling?

Specific software for geology related mapping would be, for example, Rockworks:

http://www.rockware.com/product/overview.php?id=164

Quite expensive, but relatively inexpensive for a company.

And MapInfo Professional is used extensively in geographical/geological statistical mapping, obviously.
http://www.mapinfo.com/products/applications/mapping-and-analytical-applications/mapinfo-professional

ArcGis is another professional option.

Inexpensive options for geological mapping software include:
Terrain Tools: http://www.softree.com/products/geo.htm
GRASS: http://grass.itc.it/intro/general.php

More general landscape modeling/rendering software:
http://www.geomantics.com/ (Genesis IV)
http://www.planetside.co.uk/terragen/ (Terragen, free)
http://download.cnet.com/Bryce/3000-6677_4-10696716.html (Bryce 5.5, free, must register at daz website)

The list goes on and on – have a look here for an exhaustive list:
http://www.3dlinks.com/links.cfm?categoryid=1&subcategoryid=72&CFID=21972408&CFTOKEN=90627058

A list of geo software can be found at:
http://www.geologylinks.com/Software.html

A list of open source GIS software:
http://opensourcegis.org/

Enjoy!