I would think if you can set your motherboard to keep the onboard HD3200 running while the HD4850 is inserted, then, yes, you should have three active video ports (2 on the HD4850 and one the motherboard for the HD3200).
It may not let you, in which case, the answer is no.
One-two years back, the Nvidia drivers for Windows had a bug that caused a big frame rate hit for running a second monitor while gaming, and some people got around this by adding an ATI card to run the second monitor, even a very low end one. It couldn't be an Nvidia card because the Nvidia driver would manage both GPUs, and the framerate bug manifested as long as the Nvidia driver was managing two monitors. But with an ATI card, XP let both Nvidia and ATI drivers run simultaneously, so the Nvidia driver only managed one monitor, so ... no bug. On Vista this didn't work because Vista only allowed one graphics driver. (Nvidia fixed the bug in a later driver release for Vista.)
I mention this because from it, we know driver software can manage more than one GPU. And they're both ATI so even if you have Vista, you should be able to load the ATI driver software, and have it see both GPUs.