What does 'P' mean in DDR2-5300P?

Most probably stands for "Parity". That usually means it's ECC.

P = Parity also known as ECC. It is an optional extension that can be added to RAM specs specifing registered, non-registered, ecc, buffered, etc.

http://en.wikipedia.org/wiki/DDR2_SDRAM

In addition to bandwidth and capacity variants, modules can

Optionally implement ECC, which is an extra data byte lane used for correcting minor errors and detecting major errors for better reliability. Modules with ECC are identified by an additional ECC in their designation. PC2-4200 ECC is a PC2-4200 module with ECC. Be "registered", which improves signal integrity (and hence potentially clock rates and physical slot capacity) by electrically buffering the signals at a cost of an extra clock of increased latency. Those modules are identified by an additional R in their designation, whereas non-registered (a.k.a. "unbuffered") RAM may be identified by an additional U in the designation. PC2-4200R is a registered PC2-4200 module, PC2-4200R ECC is the same module but with additional ECC. Be fully buffered modules, which are designated by F or FB and do not have the same notch position as other classes. Fully buffered modules cannot be used with motherboards that are made for registered modules, and the different notch position physically prevents their insertion.

P stands for Parity (aka ECC).