Using the 'Save for web' feature automatically takes the dpi down to 72... The optimize options in the top right of the window can be taken down as far as possible, to the point where it won't decrease visual quality.
Also, you can use the little 'Image Size' tab on the right of the 'Save for Web' pop-up to adjust the size to a reasonable one - by pixels or by %.
A tip on fixing colour if it alters a bit on the 'Save for Web' preview... the tiny little arrow directly above the preview window on the right, when clicked, will give you a whole whack of colour options. I forget what I use mostly, but those options will allow you to get the true colour you mean to acheive.
Every option possible can be achieved in the Save for Web pop-up window. This way you can save your ass because you don't have to shrink your 600dpi working file in order to save for the web, then accidentally hit 'save' and lose your work...

