Help with 2wire DSL modem/router

Hello,
I have a At&t 2wire DSL router/modem downstairs. Upstairs my new wireless laptop gets good connections and dl speeds, but not great. In the same spot my desktop is horrible. Right now I have a Dynex USB wireless connection. My desktop is saying it is getting 54.Mbps, but when tested, the speeds are really poor.

I am wondering if it is the signal or if it's the Dynex USB? This may be a dumb question but can I hook up a better wireless router to the current router/modem? Maybe a "N" router? Or should I get a booster?

In terms of the desktop, should I replace the Dynex connection with an actual wireless adapter? I am running XP on the desktop by the way. In my old house I was hardwired to the internet through cat 5 and never had a problem with speeds on the desktop so I don't think it's anything in my settings or firewall.

Thanks!