Some late additions:
Tried a 3com pci card. Didn't help.
Tried my Wireless card. Didn't help either (but I expected that).
Tried an Intel pci card. The fucker didn't work at all.
So next would be shelling out 30€ for an Intel pciEx nic. Though I'm beginning to doubt an improvement.
If other people colud be so kind and compare Local vs LAN vs Internet game performance I'd be really grateful.

Again, thx to all contributers.