Your d/l speed can depend on many things other than your LAN drivers. It could be slowed by your router, your ISP, or even by the sites you are visiting. The more people on the net then the slower things get. You may be wanting to take a race car through rush hour traffic but it just isn't going to happen the way you like. Make sure that all issues on your end are optimized but the rest relies on outside factors that you may have no control over. You could always buy another LAN card but it looks like you may be putting the cart before the horse.
Run some speed tests and then reinstall your LAN drivers. DSL Reports has plenty of tips to optimize your connection and also the speed test where you can compare your speed to other in your area. http://www.dslreports.com/