I have an old pc that is a 1.6 ghz and would like to upgrade it.

The motherboard chipset is an Intel Brookdale i845D and it looks like it uses the 478 pin. But I am not sure if it can be upgraded at all. any help would be greatly appreciated.

Hmmm are you sure it's an "845D" ? I see just about every one but "D" here: http://www.intel.com/design/chipsets/mature/index.htm

As far as if it's "replaceable" or not that's a little hard to answer from just knowing the chipset you've got. The specs on the Motherboard will determine what chips can go on it.
Another thing is if you can physically remove the chip or not. Does it look like it's in a socket?

From the general specs of the 845 it looks like you've got decent processor there.

If your system performance is an issue there may be other things you can do as well. All the 845s listed can go upto 2GB of RAM. And except for the GL and GV models it looks like they all have AGP4X slots. A nice Video Card could help out your system performance also.