After some time of tweaking, I came to the conclusion to get a new router for my home-network.
Wanting to keep the old infrastructure and compatibility to my scripts etc. - I decided to go to use the RB493AH router from routerboard. It's similar to the old RB153 - however has 9 Ethernet ports (10/100), and a way faster CPU (can be configured to run at up to 800MHz, 640MHz default). It actually took me around 15Minutes this week-end to adapt the configuration (Ethernet interfaces and firewall config where the parts to adapt) and just had to replace the older one.
Result: This router has in it's default configuration a Net-Speed on WLan of 60MBit/s against 12Mbit/s, and I could retire the spare switch which was taking care of the interconnectivity in my server room. Ethernet speed is handled by the Nuilt-in configurable switch - means the CPU is not even affected when systems in the same lan/switched ports communicate.
And last but not least - the entire system uses about 5W of power. This - is the start of Green-IT !