This is a look at the quick/informal network speed testing I did tonight, but the methods are broadly applicable to your own testing. In the video, you'll follow along as I try to decide whether I use one of my two ASRock Z68 motherboard's Realtek 8168 Gigabit ports for my ESXi service console, or go with the added dual port Intel 82571EB card's Gigabit port.
While it's not a perfect test, it shows that the Realtek is faster (>2x faster) for Service Console operations such as moving a large ISO file to a datastore. But for virtual machine throughput, which matters far more in day-to-day use, both of these NIC brands are basically both showing the same speed results. This is not a scientific benchmark, it's merely me sharing how I chose which of the devices in my lab I should use. My results are not necessarily applicable to any version of ESXi 5.0 other than build 504890 that I tested with (using it's built in default drivers), nor are they necessarily applicable to any other model Realtek or Intel NICs.
My particular test passed through 2 unmanaged Netgear GS116 Gigabit switches, using all CAT6 wiring. No jumbo frames were used, and there were no other tweaks made. The pristine virtual machine Windows 7 x64 build had no Updates, and only VMware Tools installed, with the best/optimized/fastest VMXNET 3 adapter type was chosen.