there is very little difference between any network cards.
how do you shoot the devil in the back? what happens if you miss? -verbal
In my testing, I found that the 3COMs like you have perform the best among what I tested. I didn't have the opportunity to test any Realtek based cards. I was mainly testing for 2 things: throughput, and low pings. So, I never really looked at the CPU loading aspect.
Couldn't say anything about overclocking though since I have the right divider to keep my PCI bus in spec at the FSB speed I've chosen to run with.