Ahh, duplex issues.
When I had a real job, we had a nifty Fluke network tester (was about $1000 US, size of a tricorder), and had lots of fun solving duplex mismatches between switches, networking cards and anything else that would spit out a network packet. I think tools such as tcpdump and ethereal (its got pretty GUI and I think it will run under Windoze) can give you the same info if you know where to look.
Back in the old days, auto neg was dodgey and you usually had to "force" everything to full duplex if you were after performance. So I guess the next question is how to set the duplex of a card under SUSE. I think ifconfig can do it. But it can be driver specific.