I have a Garmin Streetpilot 2720 and haven't been having any problems getting a fix but I see many people opt for an external antenna. My GPS is mounted on a Ram with a clear view to the sky, the only thing that comes close to it would be me or a windshield, both at least 2 ft away. I have sucessfully used the GPS under the dash of my car by the speedometer and never had a problem losing signal. However, I haven't really taken my GPS into the city (Iowa isn't know for its architecture) and I've got several trips coming up. Is it worth it to go to all the trouble of buying one and having even more wires to plug in?
I think the 27xx/28xx series actually does pretty good on keeping a signal in my experience. I have both the 2820 and the 478 on my bike and the 478 has a much harder time holding its signal then the 2820 does. I've been very impressed with the way the 2820 holds its gps signal. I don't get around big cities much (Atlanta once in awhile), but I can't recall how the GPS did. I have a Mio I use in the car/truck. To me the extra wires wouldn't be worth it.
Take a look at your GPS info screen. If you are not showing a good strong signal from at least 8 satellites an external antenna will help. I am using Gilsson antennas with 2 2610s and it helps, especially with initial acquisition.