Please help - Fluctuating signal from router and disconnection

My Dell laptop has a built-in wireless adapter (Intel PRO 3945ABG), so no external antenna
is present. The signal from the Netgear router has some ripple, in terms of strength,
sometimes even a dip, causing a temporary loss of connection.
Shall I buy a USB dongle ? If so, will I have to disable the wireless card ? Is it
convenient ? What is a typical gain, both for the wireless embedded card and the USB
dongle ?
Thanks,

Mike

Mike