If you get a decent speed with an Ethernet cable but a lousy one with a wireless connection, the first thing to do is to ensure that your router isn't next to any other electrical equipment. (That can make a BIG difference to download speeds).
Then try changing the channel on your router
http://documentation.netgear.com/wgt624v4/enu/202-10234-01/WGT624_RM-04-05.html
If that doesn't work, try resetting the TCP/IP stack on your laptop. (Tell us which version of Windows you're using, so that we can provide you with instructions).