2 NICs 1 Comp

Unless you have a slow router (100 Mbps), I wouldn't bother, but if you really want to do it (I presume that you'll use Gigabit NICs), you need to use a different network for the new NICs.

Assuming that your network is 192.168.0.0/24 (you didn't provide info about your current network, e.g., static IP or DHCP, etc.), use IP address 192.168.2.1 mask 255.255.255.0 on the server and IP address 192.168.2.2 mask 255.255.255.0 on the PC. If the new NIC is faster, then that might be all that you'll need to do. If it doesn't work or both NICs are the same speed, then increase the metric on the Internet NICs.

How fast are you hard disks and what interface is it going through (check the number of bytes sent and received)? Are the NICs configured for half or full duplex?

Set your new NICs to Full-Duplex and make sure that local traffic goes through them.

Transfer a large file and check on what NIC traffic increases significantly. Your question is not clear to me. There is no setting for local traffic. There are settings to force traffic to a specific NIC by changing the default route, adding an entry to LMHosts, etc. What you're trying to do isn't always as easy as it sounds if you're not familiar with networks.