Dont start with the cards in the PC/servers.
Get your data network tested. If the infrastructure isn't there, then you can spend millions (figuratively speaking) on NIC cards and see no change.
Just look for a local data engineer with a calibrated fluke tester and ask him to test your network. This will then result in printable results you can read, that show cable length, quality etc.. and more importantly SPEED!
If you need any help on this let me know as i organise such tests and installs for a living