I've played HL2 now on my own computer, and i tried several settings. But i come to the conclusion... HIGH shaders kill ur computer dude!
I have a screenshot showing the differences; its helluva lot cooler with shaders on HIGH! Anyway, on LOW i can play on pretty decent resolution (1024x76

, with HIGH shaders it will barely run on 800x600.. Without water it goes fine btw
Find the shot at
http://realbot.bots-united.com/images/hl2shader.jpg