I just looked at the ASUS website page for the P5KPL-CM motherboard:
http://usa.asus.com/product.aspx?P_ID=6nnVb6RBxd7PhGmt&content=download
It says that motherboard has:
PCIe Gigabit LAN
So, you already have the necessary hardware:
just change the speed.
If you want to add a Gigabit Ethernet adapter
to your PCI slot, then read on ...
It turns out that adding Gigabit Ethernet is relatively easy now,
especially if you still have old PCI slots available.
The following assumes that you have decided on one of
Intel's excellent Gigabit LAN adapters for PCI slots e.g.:
http://www.newegg.com/Product/Product.aspx?Item=N82E16833106121&cm_re=intel_Pro%2f1000-_-33-106-121-_-Product
Install the Gigabit LAN adapter in your PCI slot,
by following the manufacturer's instructions.
You may need to supply device drivers from
the CD-ROM supplied.
In Network Connections, right-click on that connection icon:
then Properties | Configure should bring up a window
with a number of tabs.
Under the Link tab, you should see "Speed and Duplex".
For a Gigabit LAN adapter, the speed should default to 1 Gbps.
PCI is an old standard that still works well for Gigabit Ethernet:
32 bits x 33 MHz = 1,056 Megabits per second (just enough for GbE).
MRFS