I few years back I bought a Killer M1 PCI for around $20 bucks. At the time they were still going for around $200 new. The card worked great and I used it for about a year and tested it in every way I could imagine to see if it increased FPS in games, or if it decreased CPU load. It is no faster than the basic $10 generic nic with a broadcom chip.
It was infact a good bit slower when doing transfers of large amounts of files. For example when I backed up my computer, I only backed up my important files and docs, so maybe 10-20 gigs but it was 1000s of files. That took for ever, twice as long as the onboard nic or the cheap pci nic.