In the options for the selection tool you can select "fixed size", and specify 1024x768. You can then drag the selection around.
You could also do that, then mask the layer to that. You could make the mask 50% so you can see through it too. Then once done you could use the selection wand to select the 1024x768 area within the mask and then crop. A bit of a process though, probably better just to do the selection thing above I think
Another way to crop to exact size if you weren't fussy about specific positioning of the crop is to reduce the canvas size to 1024x768.
Roger.