Here’s a Photoshop template for a page based on a grid 960 pixels wide. I have found this combination of values to be useful and start most projects with a variation of this file (the color palette I start with is different).
There are 12 grid columns, each 60 pixels wide, with a 20 pixel gutter between the columns. With 20 pixel outer borders, the page dimension is 980 pixels. Using borders of 10 pixels, the page dimension is 960 pixels. The horizontal guidelines appear every 24 pixels.
This should be compatible with Photoshop CS3 and up. If you find it useful, let me know.