You can purchase either from your above posts. PC3200 is actually DDR400 which is clocked at 200 MHz but operates at effective 400 Mhz because its DDR (double data rate). So, at the moment you have a DDR400 (PC3200) memory sitting in your motherboard. You can pair it up with another similar 512 MB one, although yes, its possible that same speed memory from different manufactures may not work together.
So, my advice get a 1 GB stick and see if both of them work together otherwise get rid of your current 512 one.
In order to check more specs about your motherboard, RAM etc. without opening up your case, use this program-
http://files1.majorgeeks.com/files/0a716fe8c7745e51a3185fc8be6ca23a/systeminfo/everesthome220.exe
For the record, DDR2 is made compatible with DDR although I haven't tried them both together yet so take it with grain of salt.