I have an Integrated "Via Rhine II" ethernet 10/100 controller on my MSI KT4V-L mobo, and honestly, it's crap. It suffers from corruption of packets on the wire; I suspect that it is having the same sort of analog problems with the PHY layer as do most onboard sound chips as well (including this board). That's probably why I get better performance from a 10/100 NIC plugged into a USB (1.1) port. YMMV.
Onboard NIC shouldn't be that bad, but in my case it was. Even if I didn't have the problems with it, I've read in many cases that it is a bit of a dog-slow chip. Just like with onboard sound, onboard LAN tends to eat more CPU cycles than a seperate PCI NIC, although that can depend highly on the chipset too. The best solution, is a high-quality integrated LAN connected to a chipset high-speed bus like Intel's CSA bus, thus not hogging PCI bus bandwidth. It kind of disgusts me though, seeing so many otherwise generally quality mobos, integrating Realtek 10/100/1000 NICs bridged to the PCI bus, with less than great performance. I think because it is now a marketing line-item on the box.