The further question of choosing a new Ram for my computer

The following infomation was found in the HP website:

Memory:

Memory Dual channel memory architecture
Two 240-pin DDR2 DIMM sockets
Supported DIMM types:
PC2-3200 (400 MHz)
PC2-4200 (533 MHz)
Non-ECC memory only, unbuffered
supports 1 GB DDR2 DIMMs
Supports up to 2 GB

That's mean I can't buy a one more 1GB ram with frequency higher than 533 ???
Is it important to mention 240-pin and DIMM when I buy it?

Thanks for answer my question.