Ok, first check your motherboard has a AGP or PCI express slot. If you were going to buy a PCI card you would just be wasting money because they are really poo.
If your motherboard has an PCI-x slot, id recommend this one
If you want to use AGP and your motherboard has the slot, id get this one
As for adding new card, from onboard, its rather simple. Boot up pc and go into BIOS. Find an option in which you can disable onboard features, LAN, sound, video etc etc.
DISABLE the intergrated, turn of pc, install new card and bam, off you go.
But remember you have to check which slots you have first before you buy anything.