I'm no expert, but I will suggest a few things which I discovered during the process of getting my LAN to work with Slack-11 and a 2.6 Kernel, which I did NOT have to do with the stock 188.8.131.52 kernel:
(BTW I am using a home-made 2.6.20 kernel and have an onboard Intel Pro 10/100 ethernet)
1.. Try to avoid using the KDE network configurator, especially after you finally get the network to work. It is good to play with to determine what addresses you should be using for the IP, Broadcast address, Gateway etc.. But afterwards, try to avoid it because it just screws up your rc.inet1 & 2 files.
2.. If/when you DO use the KDE configurator, select your distro as SuSE 7.0.. I can't explain this because I don't know enough about it, but you will see that Slack 11 is NOT in the list of compatible OS's, but SuSE 7 works for me.
3... Do you have a static IP? Prolly not if you are directly connected to the net, but if you DO, then don't use DHCPD or BootP. If at all possible, set your IP's and stuff hardcoded into the rc.inet file.
4... Do you have a /etc/resolv.conf
file? (yes it is spelled correctly) in your /etc directory? Along with this file you need a nsswitch.conf file
Slack-11 requires these files for many LAN/Ethernet functions to work. As an example, mine contains the following(resolv.conf):
All that's there is the IP's of the nameservers that My ISP uses, and the name of my LAN network here at home.
(yours may end up vastly different, or similar; dropline gnome changes these files too, keep it in mind):
hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4
With the stock distro set up originally, it did not come with one of these files, and I went through heck figuring out what was missing..
Check the Docs for the HOW-TO department which should be somewhere within your distro; Verify that your HOSTNAME and HOSTS and HOSTS.CONF are all there and correct.
Finally, while I again can't say for sure, there's a chance that the kernels you are using do NOT have the module compiled or available for the ethernet card on your motherboard (the nvidia one). Personally, I havent heard much about nvidia interfaces, though I have been spending the last 2 days setting up my nVidia AGP card for dual-screens :P what a chore.. Check thru the rc.modules file and see if there is a module there for your NIC; if there is, try modprobing it into the kernel and see if that helps; if there is not a module, you may want to look into recompiling the kernel with the support needed, providing the support exists; the kernel config menu will let you know if it has a module for that NIC..
PS- Finally again, check the Hardware Compatibility list on this site for info.
Best of Luck