My onboard is a cheapo Realtek though
This is actually a good point, never thought of it like that. There are some cheap ones on ebay though, like for $10-20. May be worth a try. Although with my luck I'll prolly end up with a Chinese knockoff.Unless the drivers for the onboard NIC are horrible, getting a dedicated card isn't going to make a heap of difference. Performance over the last few feet isn't going to have a major (if any) impact when measured against the hundreds or thousands of miles outside of your PCs control.
What were you comparing it against though?I have an Intel CT adapter. The way I tested it is I watched the same YouTube video and watched my CPU utilization (6600k). My CPU was consistently around 10% lower utilization while using the CT while watching video.
Now, that doesn't necessary mean it's going to translate to real world experience, but in certain circumstances I can see why you would want the dedicated NIC if you have both CPU intensive and network intensive activity at the same time.