Receive side scaling can affect the number of cores being used. However, if you are not maxing out the processor, using more than one core might not help you. What OS and Service Pack are you using? Do you have Intel(R) PROSet installed? If your OS supports Receive Side Scaling and you have installed Intel PROSet, you can experiment with the RSS queue size and compare RSS disabled with enabled.
What other settings have you tried changing to improve performance?
Also consider that the network adapter might not be the bottleneck. You might be running into other system performance issues that are slowing you down.