is there a way to decrease the latency of an (Realtek 8139) ethernet card in Linux?
in windows for example there's a tool called "Powerstrip" which they say is able to do so.
any ideas ?

Short answer: no, the linux drivers should be pretty good. You could probably tweak it though.

Long answer: no, what is "powerstrip"? http://www.google.com/search?q=windows+Powerstrip reveals an application for your video card and some links about how it is spyware.
Latency is a function of all the devices involved in the transaction. Reducing the latency (ping time) to google.com probably has nothing to do with your ethernet card. It could, but I think we are dealing with snake oil here. But I'm just assuming.

If that doesn't answer your question, please reply.

you helped me in the way that you confirmed my expectations, that there are no known tools...

concerning your questions:

a friend of mine uses powerstrip from http://entechtaiwan.net/util/ps.shtm

he uses this tool for two reasons:
1. system monitoring
2. it can handle some efficiency behaviors of all pci-devices (such as incr., decr. latency)

if i look into '/proc/pci' i see, that my ethernet cards parameter is set to "Latency=64" (means milli-seconds). it was all the same at my friends computer, but now he could decrease it to "Latency=16" with Powerstrip, and I just don't understand which way Powerstrip acts.

You don't notice the difference between both Latencies, except if you play internet games (such as 3D shooters), but there the difference is tremendous.

http://powertweak.sourceforge.net/

It may already be included in your distribution. Try "gpowertweak".

thanks a lot !

works fine at my place..