How can I fix my Mac-Router-Mac network? My network is amazing slow, and my question is what can I do about it.

I have done the following analysis:

Transfer from MacBookPro -> Router -> Mac Mini Intel Core 1
264,3 MB = 2114, MBit in 432 seconds. == 4,89 MBPS

Transfer from MacBookPro  -> Mac Mini Intel Core 1 (Direct network)
264,3 MB = 2114, MBit in 295 seconds. == 7,16 MBPS

Transfer from MacBookPro -> Router -> MacBook Air
90,2 MB = 721,6 MBit in 147 seconds. == 4,90 MBPS

All connections via WLAN: 54 MBit/s, WPA2, WMM, autochannel.
Router is a FRITZ!Box WLAN 3170 

So, the router sucks a bit performance, thats OK. Anyway as the router shows me that all clients are connected with 54 MBit/s, shouldn't the transfer time less? What can I do/check to increase my networks speed?

Christian
Looks closer to 802.11b than 802.11g, the speed advertised for wireless networks is confusingly not equivalent to an Ethernet cable of equivalent speed.

This guide will be informative for you:

http://www.smallnetbuilder.com/wireless/wireless-basics/31083-smallnetbuilders-wireless-faq-the-essentials

Basically buy an Apple Airport Extreme to replace your current system, and try to use the 5Ghz frequencies instead of 2.4Ghz as you are probably suffering from a lot of interference.

Example setting up with existing router: