When you select the crop tool in Photoshop, the toolbar (just below the menu bar) will change to show settings for your crop.
There are three boxes you should fill in:
* Width - the width of the image you want for your web page in pixels
* Height - the height of the image you want
* Resolution - set to 72 pixels/inch
Now when you drag the crop tool across your image, it will be constrained to the proportions you set width/height. When you complete the crop, the resulting image will be exactly the dimensions you entered.
This is the simplest approach I use and will resample the image to suit.
If you don't want any resampling to occur, select the magnifying glass tool, click the "Actual Pixels" button on the tool bar, then crop what you can see without putting values in any of the crop settings (or click the "Clear" toolbar button when the crop tool is selected to reset values you may previously of selected.)
If you have the "Info" palette visible while you do the crop, it will show the width/height of your crop selection as you drag the mouse.