force nic to 100Mb full duplex?

Hi list, I tried to search for a solution for this problem but didn't find it yet. I'm using 3C905's and 3c980's in my network. Autonegotiation sets the nics to 100Mb half duplex, which doesn't "feel" much faster then my 10Mb nics I had before. I could really use more speed. Someone knows how to accomplish this ? mazzel, Marcel