hey elg,
i think what your asking is if you can add the 1 gb to the existing 512k of 2100. if HP specs ok the use of 3200 , you can mix the speeds. iif you do,all of the ram will run at the 2100 speed. also if your machine came with 2x256 modules (as Hp did to activate dual channel) adding a single stick of ram will deactivate the dual channel. to keep the memory running in dual channel, you can add your 1GB as 2x512k, or a second 1gb stick.