"I think I have found a decent graphics card (EVGA GeForce 6200 AGP 8x w/256MB memory) for not a lot of money ($40)."
nice jump, you will enjoy the speed difference
"what is the proper procedure to change the new card for the old card in my PCLinuxOS installations?"
i haven't seen similar post recently, if i remember correctly this are the steps
change the driver to vga graphics(in pcc), the most simple and common driver before removing your video card
turn off the pc, unplug the pc power connector and the open case and remove the monitor connector
remove screw form video card, move gently the lock in the agp port(if exist) and remove the video card
connect the new one in place(this model doesn't require extra power connector but if it does, check if you have the molex adapter), put again the lock and the screw, close case and connect the monitor again
turn on the system and log in, then install the proper driver for it, nvidia 170 seems to be the right one or maybe a more recent one could work too, in synaptic should be available
if the system doesn't give you gui you can write in console XFdrake to set driver and monitor options
there is another possible solution but i don't remember it now