Yes, you can use a USB wireless adapter. Roy124 makes a good point about USB 1.1, which only supports 11mbps. However, for internet access, you're probably not going to get much more performance from your ISP anyway. My cable is 10mbps, and many ppl have much less, esp. w/ DSL.
If you happen to have a wired 10/100mbps Ethernet port, you could also consider a wireless Ethernet bridge. That would have several advantages, including avoiding any issues w/ USB, eliminating any possibility of driver conflicts (because there are no drivers), offloading all your wireless processing to the bridge, and could be reused w/ your next laptop or any other computer. The only downside is they tend to have a larger footprint making them a bit awkward for laptop purposes. But they do have “travel sized” devices that can eliminate that problem.