Your system should automatically recognize and use a discrete video card. You can fully disable the IGP in your BIOS if you desire.
I wouldn't recommend a regular pci video card. They aren't very good and unlikely to be much of an improvement on your IGP if at all.
I had the HD3200 and it is a great graphics option for the money seeing as it was built into the Mobo. If the HD4200 is anything like it just go ahead and use it for some light gaming, if your Mobo supports it overclock the GPU by about 200mhz and you'll also notice a little boost.
If you really want to do some gaming though at higher resolutions and smooth frame rates go with a PCI express graphics card. The Hybrid graphics isn't worth it for most people unless a compatible card is just given to you from a friend, but if you're looking to buy a graphics card then look at around at prices and get something decent. I just picked up a GTS 250 for 117$ from Microcenter and its a huge improvement from the IGP.