Do you need more range (distance from the router) or more speed (throughput for uploads/downloads)?
For more range you might simply play with the antennas on your router. I've been told by network technicians at the university I work as the keeping the little antennas vertical is the best strategy.
For more speed, you'll be eventually limited by the speed of your internet connection. My iMac will do 802.11n, and so I bought an 802.11n capable router to utilize it. I can stream DVD movies from my network server with it, so that's fast enough for me. If your router doesn't do N, then get one that does.