i decided to go and get another stick of 512mb DDR ram today

but it's not found. I've ran a crucial scan where i get this:
* Maximum Memory Capacity: 2048 MB
* Currently Installed Memory: 512 MB DDR PC3200
* Available Memory Slots: 1
* Number of Banks: 2
* Dual Channel Support: N.A.
* CPU Manufacturer: GenuineIntel
* CPU Family: Intel(R) Pentium(R) 4 CPU 3.06GHz Model 4, Stepping 1
* CPU Speed: 3073 MHz
the new memory i got is DDR PC3200 184-pin 400Hmz DIMM
the other stick is Nanya - 512mb DDR-400MHz CL3 PC3200u -30330
i've checked and it is in right but it just wont show up
thanks