@Saithan: Those values are normal for an 100Mb LAN connection and an 802.11g WLAN connection.
@Padul: Suggestion:
1. Install Debian Squeeze on NAS (you have the link)
2. Login into Debian and add a second IP address (
ifconfig egiga0:0 192.168.222.125 netmask 255.255.255.0 - make sure you enter it without a gateway!)
3. Add a similar IP address to one of your computers connected to your router via LAN (let's say 192.168.222.126 - no gateway either!)
4. Access the NAS using the secondary IP and check the transfer rates.
It worked for me (in fact I had to do it, since I have a Gigabit switch and a 100Mb router).
Hope it helps

Edit: Before buying that Gigabit switch, I changed the HDD (had an WD7500AADS Caviar Green, bought a ST2000DL003 Barracuda LP Green). After the replacement, my transfer rates went up a little bit, so your HDDs might have something to do with the transfer speed...