Lag in large open areas with many textures that run together is typical of "lame hardware" please dont take offense to the term. Its used to identify hardware that just isnt up to standards for the game. I'm banking that if you had a better graphics card this would not be an issue. You'll be fine in small maps like iceworld and gungame maps. If you still wish to play large open maps the only thing I can suggest since all of your hardware is proprietary try this:
http://www.invision-gaming.co.uk/Counter-Strike-Source/Caseys-Config-38.html its called casey's config it lowers all the unnecessary eye candy and what not in the game ensuring high fps.