What I am using at home
Intel Pentium 4 2.6Ghz
2 x PWLA8391G Intel PCI NIC's
When I setup this box I was originally using some very cheap NIC interfaces and not seeing the speeds I should. I replaced the cheap NIC's with the PWLA8391G and I am getting the same exact speeds. On my speed tests both user and system cpu utilization doesn't go above 30%. My memory usage looks great.
My connection is 15 down and 5 up. I planned on changing that to 25/25 in the next few days but that will wait.
One thing I note that is a bit odd. When changing the interface speed from "auto" to "100MB Full Duplex" my speed tests get much better. Then after a bit they slow right back down. Then when I go from "100MB Full Duplex" back to "auto" it does the same thing. I am using untangle behind my Verizon Fios router.