Since the D3000 doesn't have Commander mode built into the camera you will have to add a second SB-700 (or SB-800, SB-900, SB-910, or SU-800) set in Commander mode in the hotshoe of your D3000 to control your remote SB-700 using i-TTL CLS wireless flash control.
If you set your D3000 in Manual exposure mode with Manual ISO and set the built-in Speedlight in Manual flash mode. Then set your SB-700 in SU-4 (optical slave) mode. The SB-700 will then fire in sync with the D3000's shutter. Keep in mind that you will have to set the power level for both the built-in Speedlight and the Off Camera SB-700 Manually based on the the Aperture and ISO you have the camera set to and the flash to subject distance using the calculator built into the D700. You could also use the trial and error method using the histogram to dial in the correct aperture and or power level. Good Luck and Enjoy your Nikons!
Thanks, Marty, for the very informative reply although I don't like the answer. I guess I will just have to get better at photography so that I can justify a more advanced Nikon at some point. But using the SB700 on the hot shoe in bounce mode is still better than using the pop up flash, right?
You may be surprised at how well the SB700 can work in bounce mode if you learn the right techniques. Take a look at www.planetneil.com. I've improved my flash work more by using Neil's info than from any number of new bits of equipment. There's a lot to learn, but it is well worth the effort.