Edit the Makefile(in the nvnet directory) and change the -falign to a -malign
If this doesn't work then you'll probably have to upgrade your version of GCC (3.2.2 is the latest) it should support the falign function.
For the 3com nic you'll need to make a small patch to the kernel i beleive (see the link posted below for details)
Check this guide for more info on getting the ASUS A7N8X running under redhat (i know your running slackware but it might give you some helpful hints on what to do, they both come with the same stock kernel)
The udma is not enabled because the kernel does not recognise your nForce 2 IDE controller. There are a few updated kernels that aim to fix this problem it might be worth you trying one of these. Version 2.4.21-pre4-ac4 is supposed to work, but i have not been able to test it as yet. (The guide posted above provides some information about this topic)
If you don't feel like compiling your own kernel the just use hdparm to enable UDMA for your hdd (typically installed in /sbin/hdparm) Check it's man pages for more info, i'd suggest something along the lines of /sbin/hdparm -d 1 -c 1 /dev/hda (read the man pages first, that's just a suggestion)
I hope this has been of some help.