Gigabit performance: Onboard vs. PCI-E NIC

OK, I recently threw a gigabit switch into my network and attached my PCs to it as opposed to directly to my router. Both of my systems have Asus boards with onboard gigabit ethernet (nVidia chipsets).

It's definitely much faster than before, but I found that it's not nearly as fast as I think it should be. When copying 1.2G files from system to system the speed bounces around a fair bit, usually between 130-250Mb/s.

Both systems are XP SP3 with fairly new, recently defragged drives. One thought was that I was running into a disk interface bottleneck. The other was that the onboard NICs aren't as good as one would hope.

Would buying a pair of PCI/PCI-E NICs likely improve performance?