I tried this the other way round about 2 weeks ago (I have a xp32 and installed a xp64 on another, fresh HDD) The new xp64 install noticed my xp32 setup and had a good go at fecking up my internet connection and graphics drivers.
It is now an ex xp64 install and after a degree of blood, sweat and swearing I have a working connection again and am not blue screening at random....
Having said that, I know a few people here who dual boot 32 and 64 with no problems at all
So, if I was you I'd get a new HDD, set it up and install the xp32 on that.......