What is the model of the USB wireless adapter?
What type of USB does your laptop have 1.0, 1.1 or 2.0?
If you don't have USB 2.0 then that will produce a slowdown in your wireless connection. If you have a 802.11g adapter and are close to your wireless router/access point that will probably account for why your wireless connection speed says 28?Mbps and not 54Mbps. This is the speed that the router and USB wireless adapter communicate at but not the data throughput and has no bearing on your Internet speed, unless you have a very, very fast connection.
How are you measuring your download speed? Are you using a site which specialises in speed tests?
Speed Test - dslreports.com
If not, are you just downloading a file from a site and looking at it's download speed? The problem with this is it depends on
1. How far away from you the web site is and the path it has to take.
2.The upload bandwidth the site has.
3.How many people are accessing the site and are trying to download the file at the same time.
There are quite a few factors beyond the control of your ISP and although they can provide you with a download speed of 680kbs it does not guarantee that their interaction with the rest of the Internet will provide the same.