Half speed with wireless connection

I am currently using an old BEFW11S4 Linksys router to wirelessly connect 3 computers to the internet. Normal browsing is fine, but downloads and speed tests show that I'm running at around 2.5~3 mb/sec. Since my cable company's advertised speed was 6 mb/sec, I thought my speed was decent considering that cable companies often inflate their actual speeds or display the maximum speed possible. However, connecting my laptop through a wired connection on the same router shows that I get up to 5.5 mb/sec. I did some quick googling, and found that many people are experiencing speed drops on wireless, and no one seems to really know how to fix it. I tweaked around RWIN values and MTU values, tried various channels, changed my beacon interval, and various other tweaks to no avail. Interestingly enough, yesterday I was able to get the normal 5.5 mb/sec on my laptop using a wireless connection. Since I got the normal speeds on my laptop, I though that maybe it was some system configuration that was limiting my speeds on the desktops. However, when I tried testing the connection on the laptop today in the same room, it dropped back down to 2.5 mb/sec.

Anyone know what's going on?

Mains plug networking? What exactly is that? Also, it seems that some people were able to get it fixed and get full speeds from wireless, although their fixes haven't worked for me. Some of my friends also report getting full speed, or atleast 80-90% of wired from wireless connections.