Matt Laur , May 27, 2010; 11:57 a.m.
The SB-800 is (roughly!) around 60ws, the SB-600 a bit less. But those aren't very useful numbers, because those units contain reflectors and lenses, and your 300ws strobes are (usually) measures bare-bulb. But a reflector on them, and the "effective" power (over what coverage angle? ... see? tricky) changes considerably.
Best thing to do use a flash meter and get a reading from each in a real-world situation (say, with the SB-800 acting as a rim light or something, with your bigger strobe as key), and see how they square up. I mix Nikon speedlights in with far more powerful studio strobes once in a while, but it does feel a bit chaotic - not least because of some visible differences in color temperatures at different power levels.