Photoshop :: How To Reduce JPG File Size Not Pixel Dimensions

I am preparing images for the web and I really have 2 questions: one about gifs, and one about jpgs.My standard procedure is to reduce the image to the desired pixel dimensions at 600 dpiThat gives me a crisp small image. then I either use it as is if the file size is low enough (I try for under 600 kb) or convert it to a gif with the save for web and devices tool.
 
So here are my 2 questions (I will count this solved with either answer)
 
1) When I convert to a gif I have the 4 boxes: one with original size, the other 3 with options but often the options are too low res for me How do I change my 3 options to start at a higher gif res?
 
2) If I try to reduce the file size of the jpg in the image size box I set the resolution lower ( 400, 300), which lowers the pixel dimensions and the filesize, but I don't want to cahnge the pixel dimensions. And  If I reset the pixel dimensions back to the size I want them, even though it is a lower resolution the file size doesn't change.

How to reduce jpg file size using only the resolution, not pixel dimensions?  PS I have tried messing with checking and unchecking the 3 little boxes( scale styles, constrain proportions, and resample) but nothing has worked.