Windows – How to detect the computer’s RAM configuration

The best way to check for compatible memory options is with Crucial's tool -- CrucialScan.exe

The Crucial System Scanner automatically analyzes your computer memory information and suggests an upgrade that's guaranteed compatible.

It has helped me on multiple old machines (workstations and laptops).
It will also answer your question accurately for your system motherboard capabilities.

For DDR2 PC2-xxxx and the MHz figures look at this DDR2 wikipedia page.

The two things are similar ways to measure almost the same thing.
Look at the Wikipedia table and small print under it.
PC2-5300 ~ 667Mtps ~ 333MHz and,
PC2-6400 ~ 800Mtps ~ 400MHz.
For normal things, either would not make too much of a difference.
If you get a price advantage, PC2-5300 would be fine.

Here is page-2 of a long article on Dual Channel memory discussing these details.

Further on your comments, people quote frequencies differently at times (probably to make life more confusing for the buyer).
There are a lot of other references that explain these terms.
Here is one more link which has these relevant lines after the explanation.

ddr2 400 = 100mhz internal speed = 200mhz external speed = 400mhz DDR = pc2-3200
ddr2 533 = 133mhz internal speed = 266mhz external speed = 533mhz DDR = pc2-4200
ddr2 667 = 166mhz internal speed = 333mhz external speed = 667mhz DDR = pc2-5300
ddr2 800 = 200mhz internal speed = 400mhz external speed = 800mhz DDR = pc2-6400

Have you tried Crucial's scanner? I'm sure it will give you a more definitive answer. Alternatively, try booting with only the 2GB stick, and run memtest to check stability. Most linux distributions with a LiveCD come with memtest on the disc as well.