you will currently struggle to run a 4800 on your rig..
your current HD3200 is an integrated chipset & cannot be compared to a 4800.
you do have an unpopulated PCIe slot.
the HD4850 would run if you change your power supply.
a good 500w PSU with at least 30A on the 12v would be sufficient.
your CPU should be fine, there may be a slight bottleneck but nothing major.
4gb ram is also good.
what size monitor do you have....the resolution will play a big part in graphics card choice..??