294 is pretty good for a beginner. 310+ is considered a pro league. With 125 fps, of course.
The best way to improve your gap and general jumping is watching how other guys jump. From the following page you can download records of best cod4 gap jumpers:
http://www.codjumper.com/gaprecords.php
Just watch them 1000 times, and try, try, try. This is how I (and believe, everyone here) learned it. If you spend really plenty of time on gap jumping, in the worst case you will manage to do 310. Which is very good.
As to your question about the use of strafe buttons, I would say,
strafe jump is definitely not the best name for the way of jumping farther than normal in Quake, CoD and other games based on the Quake engine. This is because the key part of the strafe jump is mouse turning, not strafing. It is the turning that accelerates you and thus allows to jump farther. Strafing only makes it easier and adds like 5 extra units to the whole distance.