What slots has it got spare, that Tenda card is traditional PCI, and is a decent "crab card", with the Realtek chipset, so you can go direct to Realtek for updates.
If you haven't upgraded the integrated network drivers, I'd try
http://downloadcenter.intel.com//Det...&DwnldID=18717
And then in the advanced settings there's a slider I'd suggest pulling a click or two away from max network performance, in order to lower the rate of interrupt generation. At max, it interrupts on every packet, and may compromise overall performance due to interrupt saturation.
The latest Realtek drivers also tend to have a "max performace" option, which I suspect makes them fiercer in CPU demand, so if you go the Realtek route, I'd suggest trying with that option on and off.