802.11g is only 54Mbps, there is no 108 standard yet. 802.11n will be out next year which will be high speed (something like 100 - 300 Mbps).
Although 802.11g is 54Mbps the actual throughput is around 22Mbps so your 108 connection may give around 44Mbps in real life.
Your download speed of only 100Kbps doesn't sound right. Although the wireless connection will add a little latency you shouldn't really see a differnce with Broadband speeds. I get around 200Kbps download speed using either my 54Mbps wireless or a 100Mbps fixed connection.
Possibly you have interference from other wireless devices, try changing the channnels being used.