Alright, tested the stuff a little bit more.
I've installed the DWA-160 on a laptop with Win7 RC (build, 7100 x86) with the same WinVista-driver.
Signal is very good (the distance between the laptop and router is around 4/5 meters) and i'm getting 300 - 270Mbps all the time.
I've got a 60Mbit internet-connection and when i test my connection on www.speedtest.net, it's giving me good results:
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So, i've ordered 3x 9dBi omni directional antenna's to replace the original DIR-655 antenna's.
I think this will solve my problem, since these new stronger antenna's will give me a better signal.