You can’t.
BUT, you can create a Photoshop Action that can quickly place the guides for you on a new document.
Or you can set up a blank Photoshop document with everything the way you want it and use it as a template.
Actually, I think I found a way. Control click on "Adobe Photoshop CS" and select "Show Package Contents", then got to "Contents" > "Required" > "Default New Doc Sizes.txt" and open it in TextEdit. I simply dupicated the first DV preset, changed it to 640 x 480 and left the rest of the data alone (which added the proper guides). I don’t know if this is recommended, but it worked.
Thanks this is exactly what I need to do to create a template for Widescreen Plasma displays behind newsreaders on TV! No I just have to see if the same applies in the windows version.