For most uses all nics are pretty much the same, but there are a few differences- biggest that I know of is that the cheap ones generally use the cpu to do the network calculations (encoding and so forth) while the more expensive ones have a processor onboard that does the work.
I like 3Com best for a top-line nic, and Intel second: both are fast, reliable and fairly expensive.
Mostly I use Realtek chipped nics- CompUSA specials in fact!
They cost a LOT less and work just fine for most things.
All of the above take overclocking pretty well: I have had all of them running in rigs at 175 mhz fsb at one time or another.
As an example of a situation where a nic with onboard processing does make a difference:
I am going to try setting up a Diskless Cluster here (just 1 terminal for now) and the Cluster Server needs to have a nic that encodes for itself to send data to clients or the performance will go downhill fast. The client will be using an inexpensive nic though.