said by SHoTTa35 :20Mbps is the limit for Wireless G so if you are using that or an old router in general you wont get faster speeds EVER.
Technically, the theoretical limit for 802.11g is 54 Mbps, but depending on a variety of factors (including distance from the router and objects obstructing the signal), the actual speeds you get can vary a little or a lot. Switching to a router that supports 802.11n ( (and a device that supports it) can help, but you will probably still encounter speeds that are slower than your max potential speed.
As an example, I had Blast! so my upgrade brought me to 50 Mbps. On my wired desktop, I got results around 54 Mbps on Speedtest.net, but on my wife's Macbook Pro in the living room on 802.11n, I maxed out around 45 Mbps. Still very fast, but not the full potential realized.