Cheap NICs like Realtek utilize the CPU to do some work. Better NICs like the Intel Pro line perform more work on the NIC leaving the CPU with less to do.
When purchasing NICs, how can you tell if the it will result in lower CPU utilization than a cheap NIC? Of the ones which do reduce CPU utilization, are some better than others?
Intel is the industry gold standard for NICs. They make a very clear distinction between desktop class and server class NICs, so stick to server-class Intel NICs and you will be fine. The only other brand worth considering is Broadcom, and they are a distant second in my opinion.
Bear in mind it is the chip on the NIC you are concerned with, and there are many re-branded Intel NICs (e.g. most HP NICs are Intel-based), so it doesn't have to be Intel brand.
Here's a very detailed breakdown of all Intel's Ethernet controllers: http://www.intel.com/products/ethernet/index.htm?iid=embed_portal+hdprod_ethernet#s1=all&s2=all&s3=all
Both Intel and Broadcom make GREAT offloading NIC chips, they have little significant technical difference between them but are both very good options. What you need to understand is that the most benefit can be gained by combining these great NICs with the right drivers/stack within your OS. MS's 'Scalable Networking Pack' for Windows is far from a straightforward tick in the box and the offload options under Linux deserve dedicated investigation before commiting to.
You need to understand what the chipset on the card does. Some cards will have a hardware 'TCP/IP offload engine' (essentially an embedded computer with a TCP/IP stack). You probably want to find cards with a specification that describes this.
However, unless you have a very busy network it probably won't provide much benefit.