Unless you have an incredibly fast broadband connection, you probably won't see any improvement by going to AirPort Extreme. I haven't seen any broadband connection equipment (such as DSL or cable modems) that connect to the user with anything faster than 10BaseT-which maxes out at 10Mbps. While you can't quite get there, the theoretical max for AirPort is 11Mbps.
The 'slowness' you've noticed is probably only the slight delay that having an extra step in the line between your browser and the outside world. You'd have the same thing with Extreme, though it could be fractionally less.
There are also problems people have been reporting with AirPort Extreme base stations. It looks like Apple's implementation isn't as solid as it could be. On the other hand, the original AirPort implementation, particularly with the "dual ethernet" or Snow model base station and the current software, is mature and robust.