You have a couple issues at work here. An integrated NIC is usualy not as high quality as something from 3com, Linksys, or Netgear. The reasons for this include the fact that chipset designers can't implement the best solution into the southbridge as opposed to using a chip specifically designed to handle data loads. The drivers are also more developed by these companies too. I'm not sure about the 3com NIC on the A7N8X Delux boards, those might actually be faster due to their design and being placed off the restricted PCI bandwith might make it faster but I haven't tested it.
Also, Kazaa isn't really a good way to guage speed. What you would need to to is run a setup thourgh a lan and use a real benchmark to determine true speed.