I finally got the Edimax RTL8188CUS wireless stick of my Raspberry Pi working but I keep losing the connection after just a few minutes. I can prevent this from happening when I constantly ping the Raspberry Pi. The Pi itself seems to have no outgoing connection problems but I can't access it from my PC if I don't keep the connection busy. If I lose the connection at the moment I ping my PC from the Raspberry Pi the Pi gets accessible again in both directions.
The Raspberry Pi gets 1A power and besides the wlan stick it controls a 2 port relay. The OS is Raspbian with Xbmc prebuilt by Michael Gorven. No problems when using ethernet.